Khandogya-Upanishad: First Prapathaka, Fifth Khanda

2. “Him I sang praises to, therefore art thou my only one,” thus said Kaushitaki to his son. “Do thou revolve his rays, then thou wilt have many sons.” So much in reference to the Devas.

3. Now with reference to the body. Let a man meditate on the udgitha as the breath (in the mouth), for he goes sounding Om.

4. “Him I sang praises to, therefore art thou my only son,” thus said Kaushitaki to his son. “Do thou therefore sing praises to the breath as manifold, if thou wishest to have many sons.”

5. He who knows that the udgitha is the pranava, and the pranava the udgitha, rectifies from the seat of the Hotri priest any mistake committed by the Udgitri priest in performing the udgitha, yea, in performing the udgitha.