CHĀNDOGYA UPANISHAD · 1.1.9

तेनेयं त्रयी विद्या वर्तते ।
ओम् इत्य् आश्रावयति ।
ओम् इति शंसति ।
ओम् इत्य् उद्गायति ।
एतस्यैव अक्षरस्यापचित्यै महिम्ना रसेन ॥

teneyaṃ trayī vidyā vartate ।
om ity āśrāvayati ।
om iti śaṃsati ।
om ity udgāyati ।
etasyaiva akṣarasyāpacityai mahimnā rasena ॥

By that syllable does the threefold knowledge (the sacrifice, more particularly the Soma-sacrifice, as founded on the three Vedas) proceed. When the Adhvaryu priest gives an order, he says Om. When the Hotri priest recites, he says Om. When the Udgâtri priest sings, he says Om, --all for the glory of that syllable. The threefold knowledge (the sacrifice) proceeds by the greatness of that syllable (the vital breaths), and by its essence (the oblations).

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)