Which scriptures or Vedic hymns declare the Sun as the Supreme God (Brahman)?

Question

Most of the Indian philosophies establish hierarchies of their interest of Gods. Like There are many Lord Vishnus ( MahaVishnu- Supreme Brahman ) & Similarly many Lord Shivas ( Brahman is usually called as Maheshvar, Shiva Bhattaraka, Parameshvar, PrakAshvimarsaya etc).

Similarly, there are many Lord Suns. Supreme sun is the para Brahman according to Surya. From Aditya Hridya Stotram, He is being indirectly referred as Supreme Brahman indwelling in all.

1. एष ब्रह्मा च विष्णुश्च शिवः स्कन्दः प्रजापतिः।
   महेन्द्रो धनदः कालो यमः सोमो ह्यपां पतिः॥ ॥
   
   He is pervading in all viz., Brahma (the creator), Vishnu (the Sustainer), Shiva (the destroyer), Skanda (the son of Siva), Prajapati (progenitor of human race), the mighty Indra (lord of senses), Kubera (the God of prosperity), Kala (eternal time), Yama (the Lord of death), Soma (the moon god that nourishes), and Varuna (God of rain).

These hierarchies maintained by different philosophical systems are not mutually contradictory If I infer from KS, as the supreme who is being said in all philosophies is the same due to non duality, only naming differences.

By the way, I want to ask which scriptures or Vedic hymns declare Supreme Sun as Brahman/God directly?

Answer 1

The following Veda Mantra describes the Divine Sun as the inner soul of all that exists.

Chitram devAnAm udagAd anikam chakshur mitrasya varunasya agneh (1), AprA dyAvAprithivi antarikshma (2), surya AtmA jagatas tasthushah cha (3)

Meaning

The wonderful face of the Gods has arisen, the eye of the Mitra, Varuna and Agni (1); The Sun has filled the Heaven and the Earth and the middle space (2), He, the soul of all that moves and moves not (3).

Rig Veda 1.115.1

The following Mantra describes the Sun as the highest light.

Udvayam tamasas pari jyotih pashyanta uttaram (1), devam devatrA suryam aganma jvotir uttamam (2).

Meaning

Beholding the higher Light beyond the darkness (1), We came to the Divine Sun in the Godhead, to the highest Light of all (2).

Rig Veda 1.50.10

And, the following Mantra, from the Purusha Suktam, has Aditya or Sun as its Devata.

Hrih cha lakshmih cha patnyau (1) ahorAtre pArshve (2) nakshatrAni rupam (3) ashvinau vyAttam (4) ishtam manishAna (5) amum manishAna (6) sarvam manishana (7)||

Meaning

O Sun, Hri and Lakshmi are your consorts, (1) Day and night your sides (2), Asterisms in the sky are your form (3), The Asvins are thy mouth (4); Being such, grant me whatever i desire (5), (grant me) happiness here (6) and other objects od desire (spiritual illumination).

VAjansaneyi SamhitA 31.22

Additional commentary on the Vedic Sun (adapted from Sri Aurobindo):

Surya is the both the highest Light and the highest Truth. The light of the physical solar orb seen in the physical form or body of that Divine Sun. To the Vedic Rishis there is only one universal Deva of which all other names are alike forms and cosmic aspects. Surya is the symbol of this universal Deva. He is the soul of all that moves and moves not.

..........................

The Sun inside us and the sun outside is covered by darkness, the forces of ignorance. When, by the force of tapas, he wakes up destroying the forces of ignorance. He ascends with his seven shining horses or energies to utter ocean of higher existence. He leads us to the Truth and Immortality beyond evil and darkness.

Updating the answer (adding verses from Upanishad and passages from PurAna):

I actually missed reading (as usual) that you are asking verses from any scriptures and not just from the Vedas only.

So, here are a few verses from the Suryopanishad:

namaste Aditya |
tvame va pratyaksam karma kartAsi |
tvam-eva pratyaksam brahmAsi |
tvam-eva pratyaksam vishnurasi |
tvam-eva pratyaksam rudro’si |
tvam-eva pratyaksam rigasi |
tvam-eva pratyaksam yaju rasi |
tvam-eva pratyaksam sAmAsi |
tvam-eva pratyaksam atharvAsi | tvam-eva sarvam chhando’si |

..............

I bow to you Aditya; you are the agent himself of work, the manifest Brahma, Vishnu, Rudra, Rig, Yajur, Sama, Atharva Vedas, as well as all the chandas (Metres).

AND,

AdityAd vAyur jAyate |
AdityAd bhumir jAyate |
AdityAd āpo-jAyante | AdityAj jyotir jAyate |
AdityAd vyoma diśo jAyante |
AdityAd devA jAyante |
AdityAd vedA jAyante |
Adityo vA esa etan mandala tapati|
asAvAdityo brahma |

...............

From Aditya are born air, earth, water, fire, sky, directions, Devas, Vedas; indeed the Sun gives warmth to this sphere (Planet) ; that Sun God is Brahman,

And, as far as the PurAnas are concerned, the Bhavishya PurAna extols Surya as the Supreme Being.

At the beginning of Kalpa, Tri Murthis became victims of Ahamkaar (Self-image) and when a gigantic form of luminosity appeared on the Sky, they were taken aback as to how this huge illumination emerged from! As the Great Light approached and dazzled their vision, the personification of Surya Deva in a Virat Rupa (Collosal Form) got materialized, the Tri Murthis and Devaganas greeted it again and again with veneration.

Lord BrahmA eulogized the VirAt Swarupa of Surya Deva as follows:

Namastey Deva Devesha Sahasra kiranojjvala,
Loka Deepa Namastestu Namastey Konavallabha
Bhaskaraaya Namo nityam Khakholkaya Namo Namah
Vishnavey Kaalachakraaya Somaayaamita tejasey
Namastey Pancha Kaalaaya Indraaya Vasuretasey
Khagaaya Lokanathaaya Ekachakra rathaayacha
Jadvitaya Devaaya Shivaayaamita tejasey
Tamoghnaaya Surupaaya tejasaam nidhaye namh
Arthaya Kamarupaaya Dharmaayaamita tejasey
Mokshaaya Moksha rupaaya Suryaaya Namo Namah
Krodhalobha viheenaaya lokaanaam sthith hetavey
Shubhaya Shubharupaaya Shubhadaaya Shubhhaatmaney
Shantaaya Shantarupaaya Shanatayesmaasu vai namah
Namastey Brahmarupaaya Braahmanaaya namo namah
Brahma DevaayaBrahmarupaya Brahmaney Paramaatmaney
Brahmanye cha prasaadam cha vai kuru Deva Jatpatey

.................

Deva Devesha! Loka Deepa! Kona Vallabha! You are the source of phenominal illumination to the whole World; You are all the Deities like Bhaskara, Vishnu, Kaalachakra, Soma, Kaala, Indra, Vasu, Agni, Khaga, Lokanatha rolled in one as the Ekachakri, bestowing propitiousness and positiveness all over; You are the Embodiment of Dharma, Artha, Kama and Moksha; You are the Personification of Tranquility and Peace, devoid of anger, greed, jealousy and partiality; You are the Parabrahma Himself!

Afterwards, Lord Shiva and Lord Vishnu also eulogized Surya Deva in similar manner. You can read their stotrams to Surya on the linked page.

Answer 2

There are several. One is the Mahanarayana Upanishad. A few verses from this Upanishad (Swami Vimalananda translator):

1.2.1 The universe arose from Visvakarman through water, earth, fire, and other elements. He excelled Aditya, Indra, and other gods. The sun called Tvasta rises in the morning embodying His brilliance. In the beginning of creation the mortal world enveloped in gloom received its divine brilliance from the sun shining in the glory of the Paramatman.

1.2.4 Salutation to the resplendent Sun-God who is the son of Parabrahman [Nirguna Brahman], who shines for the benefit of gods, who is invoked as the beneficent leader of the gods, and who was born as the eldest among the gods.

1.2.6 O Sun, Hri and Laksmi are Thy consorts, Thyself being Brahma, Visnu, and Siva. Day and night are Thy two sides. Asterisms in the sky are Thy own form...

Answer 3

• In Akshi Upanishad mantra 48

अविद्यातिमिरातीतं सर्वाभासविवर्जितम् । आनन्दममलं शुद्धं मनोवाचामगोचरम्॥ प्रज्ञानघनमानन्दं ब्रह्मास्मीति विभावयेत्॥

I (Surya) am Brahman, solid Intelligence and Bliss, free from impurity, holy, lifted above mind and words, beyond the darkness of ignorance, beyond all appearances. This is the secret doctrine.

• Niralamb Upanishad

परमात्मा स ब्रह्मा स विष्णुः स इन्द्रः स शमनः स सूर्यः स चन्द्रस्ते सुरास्ते असुरास्ते पिशाचास्ते मनुष्यास्ताः स्त्रियस्ते पश्वादयस्तत्स्थावरं ते ब्राह्मणादयः । सर्वं खल्विदं ब्रह्म नेह नानास्ति किञ्चन ॥८-९॥

8-9. Brahma, Vishnu, Indra, (the god of) Death, the Sun, the Moon, the gods, the demons, men, women, animals, etc.; the immobile the Brahmanas, etc.; are that very Supereme Brahman.