Chandogya Upanishad
om ity etad akṣaram udgītham upāsīta om iti hy udgāyati tasyopavyākhyānam
Let a man meditate on the syllable Om, for the udgîtha is sung beginning with Om. And this is the full account of the syllable Om:—
devā vai mṛtyor bibhyatas trayīṃ vidyāṃ prāviśan te chandobhir acchādayan yad ebhir acchādayaṃs tacchandasāṃ chandastvam
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.
tān u tatra mṛtyur yathā matsyam udake paripaśyed evaṃ paryapaśyad ṛci sāmni yajuṣi te nu vittvordhvā ṛcaḥ sāmno yajuṣaḥ svaram eva prāviśan
Then, as a fisherman might observe a fish in the water, Death observed the Devas in the Rik, Yagus, and Sâman-(sacrifices). And the Devas seeing this, rose from the Rik, Yagus, and Sâman-sacrifices, and entered the Svara, i.e. the Om (they meditated on the Om).
yadā vā ṛcam āpnoty om ity evātisvarati evaṃ sāmaivaṃ yajuḥ eṣa u svaro yad etad akṣaram etad amṛtam abhayam tat praviśya devā amṛtā abhavan
When a man has mastered the Rig-veda, he says quite loud Om; the same, when he has mastered the Sâman 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.
sa ya etad evaṃ vidvān akṣaraṃ praṇautyetad evākṣaraṃ svaram amṛtam abhayaṃ praviśati tat praviśya yad amṛtā devās tad amṛto bhavati
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.