SB 12.6.42
tasya hy asams trayo varna
a-karadya bhrgudvaha
dharyante yais trayo bhava
guna-namartha-vrttayah
Translation:
Omkara exhibited the three original sounds of the alphabet — A, U and M. These three, O most eminent descendant of Bhrgu, sustain all the different threefold aspects of material existence, including the three modes of nature, the names of the Rg, Yajur and Sama Vedas, the goals known as the Bhur, Bhuvar and Svar planetary systems, and the three functional platforms called waking consciousness, sleep and deep sleep.