Chāndogya Upaniṣad 2.22.3

2.22.3

sarve svarā indrasyātmānaḥ \
sarva ūṣmāṇaḥ prajāpater ātmānaḥ \
sarve sparśā mṛtyor ātmānaḥ \
taṃ yadi svareṣūpālabheta \
indraṃ śaraṇaṃ prapanno 'bhūvaṃ sa tvā prati vakṣyatīty enaṃ brūyāt \\222.3\

3. All vowels are embodiments (*ātman*) of Indra. All spirants are embodiments of Prajāpati. All [other] consonants are embodiments of Mṛityu (Death).

If one should reproach a person on his vowels, let him say to that one: ‘I have been a suppliant to Indra for protection. He will answer you.’