Bhagavatapurana 12.6.42

12.6.42

tasya hy āsaṃs trayo varṇā a-kārādyā bhṛgūdvaha /
dhāryante yais trayo bhāvā guṇa-nāmārtha-vṛttayaḥ //

42. O best of Bhrigu race, three indeed are the letters that are comprised in it, A and others. Quality (Satva, Rajas and Tamas)

names (Rig, Yajus and Sama) and substance (the three worlds known as Bhur, Bhuva, Svah), wakeful and other states are conveyed by them.