Chandogya Upanishad 1.3

Now follows the meditation on the udgîtha with reference to the gods. Let a man meditate on the udgîtha (Om) as he who sends warmth (the sun in the sky). When the sun rises it sings as Udgâtri for the sake of all creatures. When it rises it destroys the fear of darkness. He who knows this, is able to destroy the fear of darkness (ignorance).

This (the breath in the mouth) and that (the sun) are the same. This is hot and that is hot. This they call svara (sound), and that they call pratyâsvara (reflected sound). Therefore let a man meditate on the udgîtha (Om) as this and that (as breath and as sun).

Then let a man meditate on the udgîtha (Om) as vyâna indeed. If we breathe up, that is prâna, the up-breathing. If we breathe down, that is apâna, the down-breathing. The combination of prâna and apâna is vyâna, back-breathing or holding in of the breath. This vyâna is speech. Therefore when we utter speech, we neither breathe up nor down.

Speech is Rik, and therefore when a man utters a Rik verse he neither breathes up nor down. Rik is Sâman, and therefore when a man utters a Sâman verse he neither breathes up nor down. Sâman is udgîtha, and therefore when a man sings (the udgîtha, Om) he neither breathes up nor down.

And other works also which require strength, such as the production of fire by rubbing, running a race, stringing a strong bow, are performed without breathing up or down. Therefore let a man meditate on the udgîtha (Om) as vyâna.

Let a man meditate on the syllables of the udgîtha, i. e. of the word udgîtha. Ut is breath (prâna), for by means of breath a man rises (uttishthati). Gî is speech, for speeches are called girah. Tha is food, for by means of food all subsists (sthita).

Ut is heaven, gî the sky, tha the earth. Ut is the sun, gî the air, tha the fire. Ut is the Sâma-veda, gî the Yagur-veda, tha the Rig-veda. Speech yields the milk, which is the milk of speech itself, to him who thus knowing meditates on those syllables of the name of udgîtha, he becomes rich in food and able to eat food.

Next follows the fulfilment of prayers. Let a man thus meditate on the Upasaranas, i. e. the objects which have to be approached by meditation: Let him (the Udgâtri) quickly reflect on the Sâman with which he is going to praise;

Let him quickly reflect on the Rik in which that Sâman occurs; on the Rishi (poet) by whom it was seen or composed; on the Devatâ (object) which he is going to praise;

On the metre in which he is going to praise; on the tune with which he is going to sing for himself;

On the quarter of the world which he is going to praise. Lastly, having approached himself (his name, family, &c.) by meditation, let him sing the hymn of praise, reflecting on his desire, and avoiding all mistakes in pronunciation, &c. Quickly will the desire be then fulfilled to him, for the sake of which he may have offered his hymn of praise, yea, for which he may have offered his hymn of praise.