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SB 12.6.42

tasya hy āsaṁs trayo varṇā
 a-kārādyā bhṛgūdvaha
dhāryante yais trayo bhāvā
tasya — of that oṁkāra; hi — indeed; āsan — came into being; trayaḥ — three; varṇāḥ — sounds of the alphabet; a-kāra-ādyāḥ — beginning with the letter a; bhṛgu-udvaha — O most eminent of the descendants of Bhṛgu; dhāryante — are sustained; yaiḥ — by which three sounds; trayaḥ — the threefold; bhāvāḥ — states of existence; guṇa — the qualities of nature; nāma — names; artha — goals; vṛttayaḥ — and states of consciousness.
Oṁkāra exhibited the three original sounds of the alphabet — A, U and M. These three, O most eminent descendant of Bhṛgu, sustain all the different threefold aspects of material existence, including the three modes of nature, the names of the Ṛg, Yajur and Sāma Vedas, the goals known as the Bhūr, Bhuvar and Svar planetary systems, and the three functional platforms called waking consciousness, sleep and deep sleep.