Chandogya Upanishad 1.1

Let a man meditate on the syllable Om, called the udgîtha; for the udgîtha (a portion of the Sâma-veda) is sung, beginning with Om. The full account, however, of Om is this:—

The essence of all beings is the earth, the essence of the earth is water, the essence of water the plants, the essence of plants man, the essence of man speech, the essence of speech the Rig-veda, the essence of the Rig-veda the Sâma-veda, the essence of the Sâma-veda the udgîtha (which is Om).

That udgîtha (Om) is the best of all essences, the highest, deserving the highest place, the eighth.

What then is the Rik? What is the Sâman? What is the udgîtha? This is the question.

The Rik indeed is speech, Sâman is breath, the udgîtha is the syllable Om. Now speech and breath, or Rik and Sâman, form one couple.

And that couple is joined together in the syllable Om. When two people come together, they fulfil each other's desire.

Thus he who knowing this, meditates on the syllable (Om), the udgîtha, becomes indeed a fulfiller of desires.

That syllable is a syllable of permission, for whenever we permit anything, we say Om, yes. Now permission is gratification. He who knowing this meditates on the syllable (Om), the udgîtha, becomes indeed a gratifier of desires.

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).

Now therefore it would seem to follow, that both he who knows this (the true meaning of the syllable Om), and he who does not, perform the same sacrifice. But this is not so, for knowledge and ignorance are different. The sacrifice which a man performs with knowledge, faith, and the Upanishad is more powerful. This is the full account of the syllable Om.