There are many Upanishads. I just want to know:
What is the source of creation according to Upanishads?
What is exactly written on the scriptures?
The source of creation has been named as Brahman in the upanishads. It is said as Brahman because it expands or increases itself and makes others increase too:
yasmācca bṛhati bṛṃhayati ca sarvaṃ tasmāducyate paraṃbrahmeti [Shandilya Up. - 3.2]
— Because He increases and caused everything to increase, it is called supreme Brahman.
It is interesting how the above definition matches with the ever expanding state of the universe. Anyway, pointing out that Brahman is the singularity and the ultimate substratum for creation, sustenance and desolation, the Taittiriya Upanishad states like this:
yato vā imāni bhūtāni jāyante yena jātāni jīvanti yatprayantyabhisaṃviśanti tadvijijñāsasva tadbrahmeti [Tait. Up. - 3.1]
That from which all these beings are born, having born by which they live, That into which having departed they enter, seek to know That, That is Brahman.
So in the beginning Brahman alone was there. Then it desired to become many. And getting heated in austerity it created a pair (matter and life) which became many beings:
brahma vā idam agra āsīt [Brh. Up. - 1.4.10]
—Brahma, the absolute, was alone at the beginning.
so 'kāmayata . bahu syāṃ prajāyeyeti [Tait. Up. - 2.6]
— He desired. "May I be many,may I grow forth"
sa tapastaptvā mithunamutpādayate rayiṃca prāṇaṃceti etau me bahudhā prajāḥ kariṣyata iti [Prsn. Up. - 1.4]
— After getting heated by austerity He produced a pair, matter and life, thinking, “These two will create many beings for Me”
And it terms of matter or the bhutas, from that single entity arose fire, water, etc. and ultimately man in the following order:
tasmādvā etasmādātmana ākāśaḥ saṃbhūtaḥ ākāśādvāyuḥ vāayoragniḥ agnerāpaḥ adabhyaḥ pṛthivī pṛthivyā oṣadhayaḥ oṣadhībhyonnam annātpuruṣaḥ [Tait. Up. - 2.1]
Verily from this Self arose the space; from space air, from air fire, from fire water, from water earth, from earth minerals, from minerals vegetation and from vegetation man.
Then having created everything, it entered into everything and that single nondual Brahman became everything that there is:
tatsṛṣṭvā tadevānuprabiśat | tadanuprabiśya sacca tyaccābhabat | niruktaṃ cāniraktaṃ ca | nilayanaṃ cānilayanaṃ ca | vijñānaṃ cāvijñānaṃ ca | satyaṃ cānṛtaṃ ca | satyaṃbhabat yadidaṃ kinca | tatsatyamityācakṣate | [Tait. Up. - 2.6]
Having created it entered into it. Having entered it became manifest and unmanifest, defined and undefined, supported and unsupported, sentient and insentient, real and unreal. The Satya became all these that there is. Hence, That is called as Satya.
So also many other upanishads name Brahman as the root cause of creation, destruction, etc. as the other answers state. But the account and description about Brahman and the creation vary to certain extent.
Some say "there is no creation in Hinduism", but that is partially wrong. In Hinduism there is both concept of creation and projection. Followers of adviata school state there is only projection. But in the statement tatsṛṣṭvā tadevānuprabiśat, the word sṛṣṭvā clearly states creation. Also in statements of Gita like visṛjāmi punaḥ punaḥ the word sṛjanā simply means creation of something new, not projection.
First, you need to be exact in your terms so there is no confusion. You asked about the sources of 'creation'. There is no creation of something out of nothing as is popularly believed in Christianity. There is no creation in Hinduism. The sanskrit word translates as projection. The universe is projected from Brahman.
The Maitrayani Upanishad (VI. 17.) says "In Me the universe has its origin, in Me alone the whole subsists, in Me it is lost: this Brahman, the Limitless--It is I Myself." Krishna says in the Gita (IX. 7.) "At the end of a cycle all beings, O Son of Kunti, enter into My prakriti, and at the beginning of a cycle I generate them again."
Swami Nikhilananda says in his introduction to his translation of the Upanishads:
"In the beginning--that is to say, before the evolution of names and forms, time and space--Atman, or Brahman, alone exists. Then it becomes conditioned by maya, Its own inscrutable power. At that time Brahman is called Saguna Brahman--Mahesvara, or the Great Lord. The idea of creation arises in His mind. Sa aikshata--"He thought." Then Brahman, on account of maya, forgets, as it were, Its infinite nature and regards Itself as an individual entity. It says 'I am one; I shall be many.'"
"Three 'moments' are to be distinguished in creation: First, the Supreme Brahman accepts the limitations of maya and becomes Mahesvara. Second, the desire for creation arises in His mind. Third, He feels His loneliness and decides to multiply Himself. Then with the help of maya, He creates akasa, air, and the other elements." [out of akasa come the other subtle elements, earth, fire, etc. Out of these subtle elements come the gross elements we identify with the material world]
"Mahesvara, who is the Ruler of all the Brahmandas, is thus the First Person in creation. Hiranyagarbha, or Brahma, who as a result of spiritual disciplines practiced in a previous cycle, becomes the Ruler of a Brahmanda, is the Second Person. Though possessed of an individuality, He identifies Himself with the whole universe; He is described in the Vedas as endowed with innumerable heads, innumerable eyes, and innumerable feet. And the Godhead dwelling in every heart is the Third Person. He is Antaryamin, or the Inner Guide."
This is a summation by Nikhilananda of Aitareya Upanishad I. i.-iii.
In his commentary to verse Aitareya II. i. 1., Sankaracarya says that all these descriptions of the 'creation' in Part 1 and Part 2 can be taken allegorically, they do not have to be taken literally. His opponent argues that how can Brahman think, etc. Sankaracarya says that the only thing sought to be taught is Knowledge of the Self (Brahman). Sankara says "Or a better explanation is that the the Deity, who is omniscient and omnipotent and is a great conjurer, created all this like a magician; but the parable etc. are elaborated here for the sake of easy instruction and comprehension just as it is done in ordinary life. For the mere acquaintance with the anecdotes regarding creation etc. leads to no useful result, whereas it is well known in all the Upanishads that from the knowledge of the unity of the Self follows immortality as a result, and the same fact is in evidence in the Smritis like the Gita in such sentences as '(He sees who sees), the Lord Supreme, existing in all beings, (deathless in the dying)' (Gita XIII. 27.)"
If you want to know what the source of creation is according to the Upanishads, that can be answered in a single word: Brahman. In fact, that is how Varuna the ocean god explains Brahman to the sage Bhrigu in the Third Valli of the Taittirya Upanishad:
Bhrigu Vâruni went to his father Varuna, saying: "Sir, teach me Brahman." He told him this, viz. Food, breath, the eye, the ear, mind, speech. Then he said again to him: 'That from whence these beings are born, that by which, when born, they live, that into which they enter at their death, try to know that. That is Brahman.'
(I'm not clear on why Varuna is referred to as Bhrigu's father, which is why I asked this question.). This is the quotation that Vyasa uses in the beginning of his Brahma Sutras to define Brahman (see Sutra 2 of the Brahma Sutras here).
But I suspect that what you really want is not just the source of creation, but rather what the story of creation is according to the Upanishads. Well, three of the oldest Upanishads have famous creation accounts, each focusing on different aspects of creation:
The initial chapters of the Brihadaranyaka Upanishad contain a creation account copied word-for-word from Book 10 of the Shatapatha Brahmana of the Yajur Veda. In particular, here is what the Agni Brahmana of the Brihadaranyaka Upanishad says:
In the beginning there was nothing (to be perceived) here whatsoever. By Death indeed all this was concealed,--by hunger; for death is hunger. Death (the first being) thought, 'Let me have a body.' Then he moved about, worshipping. From him thus worshipping water was produced.... And what was there as the froth of the water, that was hardened, and became the earth. On that earth he (Death) rested, and from him, thus resting and heated, Agni (Virâg) proceeded, full of light. That being divided itself threefold, Âditya (the sun) as the third, and Vâyu (the air) as the third. That spirit (prâna) became threefold. The head was the Eastern quarter, and the arms this and that quarter (i. e. the N. E. and S. E., on the left and right sides). Then the tail was the Western quarter, and the two legs this and that quarter (i. e. the N. W. and S. W.) The sides were the Southern and Northern quarters, the back heaven, the belly the sky, the dust the earth. Thus he (Mrityu, as arka) stands firm in the water... He desired 'Let a second body be born of me,' and he (Death or Hunger) embraced Speech in his mind. Then the seed became the year. Before that time there was no year. Speech bore him so long as a year, and after that time sent him forth. Then when he was born, he (Death) opened his mouth, as if to swallow him. He cried Bhân! and that became speech He thought, 'If I kill him, I shall have but little food.' He therefore brought forth by that speech and by that body (the year) all whatsoever exists, the Rik, the Yagus, the Sâman, the metres, the sacrifices, men, and animals....
He desired to sacrifice again with a greater sacrifice. He toiled and performed penance. And while he toiled and performed penance, glorious power went out of him. Verily glorious power means the senses (prâna). Then when the senses had gone out, the body took to swelling (sva-yitum), and mind was in the body. He desired that this body should be fit for sacrifice (medhya), and that he should be embodied by it. Then he became a horse (asva), because it swelled (asvat), and was fit for sacrifice (medhya)... Then, letting the horse free, he thought, and at the end of a year he offered it up for himself, while he gave up the (other) animals to the deities. Verily the shining sun is the Asvamedha-sacrifice, and his body is the year; Agni is the sacrificial fire (arka), and these worlds are his bodies. These two are the sacrificial fire and the Asvamedha-sacrifice, and they are again one deity, viz. Death. He (who knows this) overcomes another death, death does not reach him, death is his Self, he becomes one of those deities.
The Purushavidha Brahmana of the Brihadaranyaka describes the creation of living things, but above is the description of the creation of the Universe.
The Chandogya Upanishad discusses creation in two places. First, here is what it says in the Third Prapathaka:
In the beginning this was non-existent. It became existent, it grew. It turned into an egg. The egg lay for the time of a year. The egg broke open. The two halves were one of silver, the other of gold. The silver one became this earth, the golden one the sky, the thick membrane (of the white) the mountains, the thin membrane (of the yoke) the mist with the clouds, the small veins the rivers, the fluid the sea. And what was born from it that was Âditya, the sun. When he was born shouts of hurrah arose, and all beings arose, and all things which they desired.
And here is what it says in the Sixth Prapathaka:
How could that which is, be born of that which is not? No, my dear, only that which is, was in the beginning, one only, without a second. It thought, may I be many, may I grow forth. It sent forth fire. That fire thought, may I be many, may I grow forth. It sent forth water.... Water thought, may I be many, may I grow forth. It sent forth earth (food).... That Being (i. e. that which had produced fire, water, and earth) thought, let me now enter those three beings 5 (fire, water, earth) with this living Self (gîva âtmâ), and let me then reveal (develop) names and forms. 'Then that Being having said, Let me make each of these three tripartite (so that fire, water, and earth should each have itself for its principal ingredient, besides an admixture of the other two) entered into those three beings (devatâ) with this living self only, and revealed names and forms.
Here is what the First Khanda of the Aitareya Upanishad says:
Verily, in the beginning all this was Self, one only; there was nothing else blinking 2 whatsoever. He thought: 'Shall I send forth worlds?' He sent forth these worlds, Ambhas (water), Marîki (light), Mara (mortal), and Ap (water). That Ambhas (water) is above the heaven, and it is heaven, the support. The Marîkis (the lights) are the sky. The Mara (mortal) is the earth, and the waters under the earth are the Ap world. He thought: 'There are these worlds; shall I send forth guardians of the worlds?' He then formed the Purusha (the person), taking him forth from the water. He brooded on him, and when that person had thus been brooded on, a mouth burst forth 4 like an egg. From the mouth proceeded speech, from speech Agni (fire) Nostrils burst forth. From the nostrils proceeded scent (prâna), from scent Vâyu (air). Eyes burst forth. From the eyes proceeded sight, from sight Âditya (sun). Ears burst forth. From the ears proceeded hearing, from hearing the Dis (quarters of the world), Skin burst forth. From the skin proceeded hairs (sense of touch), from the hairs shrubs and trees. The heart burst forth. From the heart proceeded mind, from mind Kandramas (moon). The navel burst forth. From the navel proceeded the Apâna (the down-breathing), from Apâna death. The generative organ burst forth. From the organ proceeded seed, from seed water.