Taittiriyaka-Upanishad: First Valli, Fifth Anuvaka
1. Bhu, Bhuvas, Suvas, these are the three sacred interjections (vyahriti). Mahakamasya taught a fourth, viz. Mahas, which is Brahman, which is the Self. The others (devatas) are its members.
Bhu is this world, Bhuvas is the sky, Suvas is the other world.
2. Mahas is the sun. All the worlds are increased by the sun. Bhu is Agni (fire), Bhuvas is Vayu (air), Suvas is Aditya (sun). Mahas is the moon. All the heavenly lights are increased by the moon.
Bhu is the Rik-verses, Bhuvas is the Saman-verses, Suvas is the Yagus-verses.
3. Mahas is Brahman. All theVedas are increased by the Brahman.
(1-2) Bhu is Prâna (up-breathing), Bhuvas is Apana (down-breathing), Suvas is Vyana (backbreathing). Mahas is food. All breathings are increased by food.
Thus there are these four times four, the four and four sacred interjections. He who knows these,
(1-2) Knows the Brahman. All Devas bring offerings to him.