Markandeyapurana 42.11

42.11

savyañjano makāraśca svarlokaḥ parikalpyate /
vyaktā tu prathamā mātrā dvitīyāvyaktasaṃjñitā //

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.