Chāndogya Upaniṣad 2.22.3

All vowels (svara) belong to Indra, all sibilants (ushman) to Pragapati, all consonants (sparsa) to Mrityu (death). If somebody should reprove him for his vowels, let him say, `I went to Indra as my refuge (when pronouncing my vowels): he will answer thee.'