Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 9

Sanskrit:

दिशां त्वमवकाशोऽसि दिश: खं स्फोट आश्रय: ।नादो वर्णस्त्वम् ॐकार आकृतीनां पृथक्कृति: ॥ ९ ॥

ITRANS:

diśāṁ tvam avakāśo ’sidiśaḥ khaṁ sphoṭa āśrayaḥnādo varṇas tvam oṁkāraākṛtīnāṁ pṛthak-kṛtiḥ

Translation:

You are the directions and their accommodating capacity, the all-pervading ether and the elemental sound residing within it. You are the primeval, unmanifested form of sound; the first syllable, om; and audible speech, by which sound, as words, acquires particular references.

Purport:

In accordance with the general process of creation, speech always becomes audible in stages, which proceed from subtle inner impulse to outward expression. These stages are mentioned in the mantras of the Ṛg Veda (1.164.45): “Discriminating brāhmaṇas know of four progressive stages of language. Three of these remain hidden within the heart as imperceptible vibrations, while the fourth stage is what people ordinarily understand as speech.”