structural:reformatting, structural:expansion
Vedic literature

One of the earliest mentions of Brahma with Vishnu and Shiva is in the fifth Prapathaka (lesson) of the Maitrayaniya Upanishad, probably composed around the late 1st millennium BCE. Brahma is first discussed in verse 5,1, also called the Kutsayana Hymn, and then expounded in verse 5,2.

In the pantheistic Kutsayana Hymn, the Upanishad asserts that one's Soul is Brahman, and this Ultimate Reality, Cosmic Universal or God is within each living being. It equates the atman (Soul, Self) within to be Brahma and various alternate manifestations of Brahman, as follows, "Thou art Brahma, thou art Vishnu, thou art Rudra (Shiva), thou art Agni, Varuna, Vayu, Indra, thou art All."

In verse (5,2), Brahma, Vishnu, and Shiva are mapped into the theory of Guṇa, that is qualities, psyche and innate tendencies the text describes can be found in all living beings. This chapter of the Maitri Upanishad asserts that the universe emerged from darkness (tamas), first as passion characterized by innate quality (rajas), which then refined and differentiated into purity and goodness (sattva). Of these three qualities, rajas are then mapped to Brahma, as follows:

Now then, that part of him which belongs to tamas, that, O students of sacred knowledge (Brahmacharins), is this Rudra. That part of him which belongs to rajas, that O students of sacred knowledge, is this Brahma. That part of him which belongs to sattva, that O students of sacred knowledge, is this Vishnu. Verily, that One became threefold, became eightfold, elevenfold, twelvefold, into infinite fold. This Being (neuter) entered all beings, he became the overlord of all beings. That is the Atman (Soul, Self) within and without – yea, within and without!

While the Maitri Upanishad maps Brahma with one of the elements of the guṇa theory of Hinduism, the text does not depict him as one of the trifunctional elements of the Hindu Trimurti idea found in later Puranic literature.