Khandogya-Upanishad: Third Prapathaka, Fourteenth Khanda
1. All this is Brahman (n.) Let a man meditate on that (visible world) as beginning, ending, and breathing in it (the Brahman).
Now man is a creature of will. According to what his will is in this world, so will he be when he has departed this life. Let him therefore have this will and belief:
2. The intelligent, whose body is spirit, whose form is light, whose thoughts are true, whose nature is like ether (omnipresent and invisible), from whom all works, all desires, all sweet odours and tastes proceed; he who embraces all this, who never speaks, and is never surprised,
3. He is my self within the heart, smaller than a corn of rice, smaller than a corn of barley, smaller than a mustard seed, smaller than a canary seed or the kernel of a canary seed. He also is my self within the heart, greater than the earth, greater than the sky, greater than heaven, greater than all these worlds.
4. He from whom all works, all desires, all sweet odours and tastes proceed, who embraces all this, who never speaks and who is never surprised, he, my self within the heart, is that Brahman (n.) When I shall have departed from hence, I shall obtain him (that Self). He who has this faith has no doubt; thus said Sandilya, yea, thus he said.