Markandeyapurana 42.10

42.10

tatra yuktastu yo yogī sa tallayamavāpnuyāt /
akārastvatha bhūrloka ukāraścocyate bhuvaḥ //

Moreover the letter A is designated the bhūrloka, or terrestrial world; and the letter U the bhuvarloka, or atmospheric world; and the letter M with its nasal mark is decided to be the svarloka, or celestial world. Now the first instant is called the discrete,[4] and the second the indiscrete, and the third instant is the intellectual faculty;[5] the half instant is the highest abode.[6] In this very order must these stages of religious meditation be known. By uttering the word Om, everything both existent and non-existent may be grasped. Now the first instant is short, the second is long, and the third is prolated, and the half instant is not cognisant to speech.

The syllable Om, consisting of three and a half instants, should be known in its true sense as the three Vedas—the Ṛc, Sāma and Yajus—the three worlds, the three fires, and the three deities Viṣṇu, Brahmā and Śiva. And the yogi, who is absorbed in religious meditation thereon, may obtain extinction therein.