Padma Purana declares Sada Shiva to be originator of Brahma, Vishnu and Maheshwor in Patala Khanda chapter 108 while describing origin of Sacred ash:
Sri Rama said:
1. O illustrious one, tell about the origin of the sacred
ash, the greatness of the sacred ash, and the religious merit (earned) through the application of the sacred ash.
2-8a. O Rama, I shall tell you about the origin of the
sacred ash. (I shall tell you about its effect) by (just) remembering or telling about it. O king, listen to it. He who is that eternal god Sadasiva, who is saluted by Brahma, who has three eyes, who is the prop of virtues, who is beyond qualities, who is unchangeable and immutable, had once a desire to create on
seeing the three qualities in himself—this triad of qualities should be known as the three Vedas. O dear one, having divided himself and the region there, he created on his right side the son, viz. Brahma and Hari from his left side. At the back side he created Maheswara (i.e. Shiva); thus the mighty one created
three sons. As soon as they were born they became the three gods—Brahma, Vishnu and Shiva.
They distinctly spoke these words: "Who are you? Who are we?"
To those sons Shiva said: "You are the sons, (and) I am your father. O sons, resort to these three qualities which are the cause of (all) acts."
The sons said:
8b-16. O lord, who should resort to which quality and how long? How can there be the cessation from the qualities? Tell this to us.
As long as you have (i.e. retain your) knowledge or as long as you live, each one of these qualities should be retained by you. Brahma should retain the quality of Sattva (i.e. goodness), Visnu should retain Rajas (i.e. activity) and Mahesvara (should keep) Tamas (i.e. ignorance or darkness).
As soon as the lord of gods had said this, Brahma took up Sattva. He could not move it; then how could he be capable of holding it? Abandoning that quality, he took up the quality of
Rajas. He was not able to move it; (so) he took up the Tamas quality. He was not able to move it. He fell down and wept; and Visnu held the Rajas quality with his left hand. (Then) Mahesa held the Tamas quality with his two fingers; one, i.e. Visnu, with his two fingers, took up the Sattva (quality). He
sustained Brahma at the footstool, and danced. The giver of the boons, having seen Siva who was dancing, who had a graceful form, whose form was (fair) like the cow's milk, who was young and had three eyes, who held everything, who caused joy, said to his sons:
17-18. O son, I am pleased (with you). Ask for a boon as desired (by you).
Then the son said to the father: "Give this boon tome:
O auspicious one, let the worship intended for me be yours. You should always remain in me; you, the immutable one, are me only."
19-20. O magnanimous one, this will happen; there is no doubt. These two reddish and fair, Brahma and Visnu are my two sons. They are the fine soft hair of the armpits, they have my form and are innocent.
Then he said to Brahma: "You resort to (i.e. take) this one quality."
21-22a. O lord, I am not able to hold the quality indicated by you. O god, I shall sustain the Rajas (quality); let Hari resort to the Sattva (quality). This god will hold the remaining quality.
22b-24. After having taken the qualities, those gods were not able always to hold them. For being able to sustain (them) they again said to Siva (i.e. the lord): "We are not able to hold these qualities for all the time. O lord if you are the giver of boons, (then) give us the capacity (to hold them)." Then hearing their words, Siva said (these) words.
25-28. Of all (powers), the power of knowledge (alone) is called power. Vidya (i.e. knowledge) resorts to the three qualities. Ignorance also resorts to them. Please hold the essence of the three qualities after burning them. You should hold that what would be (remaining) there.
Then his son said (these) words: "There cannot be burning without fire."
Shiva said: "Fire certainly exists in the eye of
Sankara. These three qualities are the cow; and knowledge would be(i e. is) the auspicious cowdung. Upanisad is said to be the urine (of the cow). Then one should prepare the sacred ash from them.