Chandogya Upanishad 1.2

When the Devas and Asuras struggled together, both of the race of Pragâpati, the Devas took the udgîtha (Om), thinking they would vanquish the Asuras with it.

They meditated on the udgîtha (Om) as the breath (scent) in the nose, but the Asuras pierced it (the breath) with evil. Therefore we smell by the breath in the nose both what is good-smelling and what is bad-smelling. For the breath was pierced by evil.

Then they meditated on the udgîtha (Om) as speech, but the Asuras pierced it with evil. Therefore we speak both truth and falsehood. For speech is pierced by evil.

Then they meditated on the udgîtha (Om) as the eye, but the Asuras pierced it with evil. Therefore we see both what is sightly and unsightly. For the eye is pierced by evil.

Then they meditated on the udgîtha (Om) as the ear, but the Asuras pierced it with evil. Therefore we hear both what should be heard and what should not be heard. For the ear is pierced by evil.

Then they meditated on the udgîtha (Om) as the mind, but the Asuras pierced it with evil. Therefore we conceive both what should be conceived and what should not be conceived. For the mind is pierced by evil.

Then comes this breath (of life) in the mouth. They meditated on the udgîtha (Om) as that breath. When the Asuras came to it, they were scattered, as (a ball of earth) would be scattered when hitting a solid stone.

Thus, as a ball of earth is scattered when hitting on a solid stone, will he be scattered who wishes evil to one who knows this, or who persecutes him; for he is a solid stone.

By it (the breath in the mouth) he distinguishes neither what is good nor what is bad-smelling, for that breath is free from evil. What we eat and drink with it supports the other vital breaths (i. e. the senses, such as smell, &c.) When at the time of death he does not find that breath (in the mouth, through which he eats and drinks and lives), then he departs. He opens the mouth at the time of death (as if wishing to eat).

Angiras meditated on the udgîtha (Om) as that breath, and people hold it to be Angiras, i. e. the essence of the members (angânâm rasah);

Therefore Brihaspati meditated on udgîtha (Om) as that breath, and people hold it to be Brihaspati, for speech is brihatî, and he (that breath) is the lord (pati) of speech;

Therefore Ayâsya meditated on the udgîtha (Om) as that breath, and people hold it to be Ayâsya, because it comes (ayati) from the mouth (âsya);

Therefore Vaka Dâlbhya knew it. He was the Udgâtri (singer) of the Naimishîya-sacrificers, and by singing he obtained for them their wishes.

He who knows this, and meditates on the syllable Om (the imperishable udgîtha) as the breath of life in the mouth, he obtains all wishes by singing. So much for the udgîtha (Om) as meditated on with reference to the body.