The works of Ādi Śaṅkarācārya

Chāndogyopaniṣadbhāṣyam

Verse 6

तेनेयं त्रयीविद्या वर्तते ओमित्याश्रावयत्योमिति शंसत्योमित्युद्गायत्येतस्यैवाक्षरस्यापचित्यै महिम्ना रसेन ॥ ९ ॥

In plain words

By this means the threefold sacred lore operates: uttering 'Om' he gives the call, uttering 'Om' he recites, and uttering 'Om' he chants the Udgitha—all as an act of reverence to this very imperishable syllable, through its majesty and its essence.

Close to the Sanskrit

By that this threefold knowledge proceeds: with 'Om' he gives the call, with 'Om' he recites, with 'Om' he chants the Udgitha—for the worship of this very imperishable syllable, through its majesty and its essence.