Taittiriyaka-Upanishad: First Valli, Sixth Anuvaka
Between the two palates there hangs the uvula, like a nipple—that is the starting-point of Indra (the lord). Where the root of the hair divides, there he opens the two sides of the head, and saying Bhu, he enters Agni (the fire); saying Bhuvas, he enters Vayu (air);
2. Saying Suvas, he enters Aditya (sun); saying Mahas, he enters Brahman. He there obtains lordship, he reaches the lord of the mind. He becomes lord of speech, lord of sight, lord of hearing, lord of knowledge. Nay, more than this. There is the Brahman whose body is ether, whose nature is true, rejoicing in the senses (prâna), delighted in the mind, perfect in peace, and immortal.
(1) Worship thus, O Prakinayogya!