CHĀNDOGYA UPANISHAD · 1.4.4

यदा वा ऋचम् आप्नोत्य् ओम् इत्य् एवातिस्वरति एवं सामैवं यजुः ।
एष उ स्वरो यद् एतद् अक्षरम् एतद् अमृतम् अभयम् ।
तत् प्रविश्य देवा अमृता अभवन् ॥

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.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)