Chandogya Upanishad 2.8

atha saptavidhasya vāci saptavidhaṃ sāmopāsīta yat kiṃca vāco hum iti sa hiṅkāraḥ yat pra iti sa prastāvaḥ yad ā iti sa ādiḥ

Next for the sevenfold Sâman. Let a man meditate on the sevenfold Sâman in speech. Whenever there is in speech the syllable hun that is hinkâra, pra is the prastâva, â is the âdi, the first, i. e. Om,

yad ud iti sa udgīthaḥ yat pratīti sa pratihāraḥ yad upeti sa upadravaḥ yan ni iti tan nidhanam

Ud is the udgîtha, pra the pratihâra, upa the upadrava, ni the nidhana.

dugdhe 'smai vāg dohaṃ yo vāco dohaḥ annavān annādo bhavati ya etad evaṃ vidvān vāci saptavidhaṃ sāmopāste

Speech yields the milk, which is the milk of speech itself, to him who knowing this meditates on the sevenfold Sâman in speech. He becomes rich in food, and able to eat food.