Khandogya-Upanishad: First Prapathaka, Fourth Khanda
1. Let a man meditate on the syllable Om, for the udgitha is sung beginning with Om. And this is the full account of the syllable Om:—
2. The Devas, being afraid of death, entered upon (the performance of the sacrifice prescribed in) the threefold knowledge (the three Vedas). They covered themselves with the metrical hymns. Because they covered (khad) themselves with the hymns, therefore the hymns are called khandas.
3. Then, as a fisherman might observe a fish in the water, Death observed the Devas in the Rik, Yagus, and Saman-(sacrifices). And the Devas seeing this, rose from the Rik, Yagus, and Saman-sacrifices, and entered the Svara, i.e. the Om (they meditated on the Om).
4. When a man has mastered the Rig-veda, he says quite loud Om; the same, when he has mastered the Saman and the Yagus. This Svara is the imperishable (syllable), the immortal, free from fear. Because the Devas entered it, therefore they became immortal, and free from fear.
5. He who knowing this loudly pronounces (pranauti) that syllable, enters the Same (imperishable) syllable, the Svara, the immortal, free from fear, and having entered it, becomes immortal, as the Devas are immortal.