Taittiriyaka-Upanishad: Second Valli, Sixth Anuvaka
He who knows the Brahman as non-existing, becomes himself non-existing. He who knows the Brahman as existing, him we know himself as existing. The embodied Self of this (bliss) is the same as that of the former (understanding).
Thereupon follow the questions of the pupil:
“Does any one who knows not, after he has departed this life, ever go to that world? Or does he who knows, after he has departed, go to that world?”
The answer is: He wished, may I be many, may I grow forth. He brooded over himself (like a man performing penance). After he had thus brooded, he sent forth (created) all, whatever there is. Having sent forth, he entered into it. Having entered it, he became sat (what is manifest) and tyat (what is not manifest), defined and undefined, supported and not supported, (endowed with) knowledge and without knowledge (as stones), real and unreal. The Sattya (true) became all this whatsoever, and therefore the wise call it (the Brahman) Sattya (the true).
On this there is also this Sloka: