Taittirīya Upaniṣad 1.6.1

There is the ether within the heart, and in it there is the Person (purusha) consisting of mind, immortal, golden. 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);