Bhagavatapurana 10.85.9

10.85.9

diśāṃ tvam avakāśo 'si diśaḥ khaṃ sphoṭa āśrayaḥ /
nādo varṇas tvam oṃ-kāra ākṛtīnāṃ pṛthak-kṛtiḥ //

9. Thou art the space that constitutes the cardinal points: Thou art the support, the principle of sound and the sky and speech, Thou art the sound, the letter. Thou art the sacred syllable, that which separates the different forms, i. e., the source of different names.