Chāndogya Upaniṣad 1.1.8

1.1.8

tad vā etad anujñākṣaram \
yad dhi kiṃcānujānāty om ity eva tad āha \
eṣo eva samṛddhir yad anujñā \
samardhayitā ha vai kāmānāṃ bhavati ya etad evaṃ vidvān akṣaram udgītham upāste \\101.8\

8. Verily, this syllable is assent; for whenever one assents to anything he says simply ‘Om.’ This, indeed, is fulfilment—that is, assent is.

A fulfiller of desires, verily, indeed, becomes he who, knowing this thus, reverences the Udgītha as this syllable.