praśnopaniṣad - pañca praśnaḥ

The Fifth Question. Then Satyakama, son of Shibi, asked him: "O Venerable One, if a person among men were to meditate on the syllable 'Om' until the end of his life, which world would he win by that?"

He replied: "O Satyakama, this syllable Om is indeed both the Higher Brahman and the lower Brahman. Therefore, the wise one attains the one or the other by this means alone."

"If he meditates upon only the first element (A), being enlightened by that, he quickly attains the world of men. The verses of the Rig Veda lead him there; endowed with austerity, celibacy, and faith, he experiences greatness."

"If he meditates upon the two elements (A and U), he is led by the verses of the Yajur Veda to the intermediate space, the world of the Moon. Having experienced majesty in the lunar world, he returns again."

"But he who meditates upon the Supreme Person by means of the three elements of this syllable Om, he is united with the light of the Sun. Just as a snake is freed from its slough, so he is freed from sin and is led by the verses of the Sama Veda to the world of Brahman. There, he beholds the Person who dwells in the city of the body, who is higher than the highest life-force. Regarding this, there are these two verses:"

"The three elements, when employed singly, are subject to death. However, when they are closely connected and not separate—being well-applied in external, internal, and intermediate actions—the wise man does not tremble."

"The wise man reaches this world through the Rig Veda, the intermediate space through the Yajur Veda, and that which is known to the seers through the Sama Veda. By this means of Om, the wise man reaches that which is tranquil, ageless, deathless, fearless, and supreme."