Srimad Bhagavatam Canto 10, Chapter 85, Text 09

SB 10.85.9

disam tvam avakaso ’si
 disah kham sphota asrayah
nado varnas tvam omkara
 akrtinam prthak-krtih
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.
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 Rg Veda (1.164.45):
catvari vak-parimita padani
 tani vidur brahmana ye manisinah
guhayam trini nihitani nengayanti
 turiyam vaco manusya vadanti
“Discriminating brahmanas 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.”
Srimad Bhagavatam Canto 10, Chapter 85, Text 08
Srimad Bhagavatam Canto 10, Chapter 85, Text 10