Markandeyapurana 42.14

42.14

tṛtīyā ca plutārdhākhyā vacasaḥ sā na gocarā /
ityetadakṣaraṃ brahma paramoṅkārasaṃjñitam //

Such is this word. Brahma is designated the Supreme “Om.” The man who truly understands it and further meditates on it, escaping the circle of mundane existence casts off the three-fold bonds, and gains sublime extinction in Brahma, the Supreme Soul. And he who is bound with the unconsumed results of his actions, after experiencing death through ill omens,[7] and recollecting it at the time of his departure, attains to a yogi’s condition again. Hence by means of imperfect religious devotion, or again by perfected religious devotion, are always to be known the ill omens, so that be does not sink into despondency at the time of his departure.

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.