Chandogya Upanishad 1.5

atha khalu ya udgīthaḥ sa praṇavo yaḥ praṇavaḥ sa udgītha iti asau vā āditya udgītha eṣa praṇavaḥ om iti hy eṣa svarann eti

The udgîtha is the pranava, the pranava is the udgîtha. And as the udgîtha is the sun, so is the pranava, for he (the sun) goes sounding Om.

etam u evāham abhyagāsiṣaṃ tasmān mama tvam eko 'sīti ha kauṣītakiḥ putram uvāca raśmīṃs tvaṃ paryāvartayāt bahavo vai te bhaviṣyanti ity adhidaivatam

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

athādhyātmam ya evāyaṃ mukhyaḥ prāṇas tam udgītham upāsīta om iti hy eṣa svarann eti

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

etam u evāham abhyagāsiṣaṃ tasmān mama tvam eko 'sīti ha kauṣītakiḥ putram uvāca prāṇāṃs tvaṃ bhūmānam abhigāyatād bahavo vai me bhaviṣyantīti

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

atha khalu ya udgīthaḥ sa praṇavo yaḥ praṇavaḥ sa udgītha iti hotṛṣadanāddha evāpi durudgītham anusamāharatīty anusamāharatīti

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