Prashna Upanishad 5

Then Saivya Satyakâma asked him: "Sir, if some one among men should meditate here until death on the syllable Om, what would he obtain by it?"

He replied: "O Satyakâma, the syllable Om (AUM) is the highest and also the other Brahman; therefore he who knows it arrives by the same means at one of the two.

If he meditate on one Mâtrâ (the A), then, being enlightened by that only, he arrives quickly at the earth. The Rik-verses lead him to the world of men, and being endowed there with penance, abstinence, and faith, he enjoys greatness.

If he meditate with two Mâtrâs (A+U) he arrives at the Manas, and is led up by the Yagus-verses to the sky, to the Soma-world. Having enjoyed greatness in the Soma-world, he returns again.

Again, he who meditates with this syllable AUM of three Mâtrâs, on the Highest Person, he comes to light and to the sun. And as a snake is freed from its skin, so is he freed from evil. He is led up by the Sâman-verses to the Brahma-world; and from him, full of life (Hiranyagarbha, the lord of the Satya-loka), he learns to see the all-pervading, the Highest Person. And there are these two Slokas:

The three Mâtrâs (A+U+M), if employed separate, and only joined one to another, are mortal; but in acts, external, internal, or intermediate, if well performed, the sage trembles not.

Through the Rik-verses he arrives at this world, through the Yagus-verses at the sky, through the Sâman-verses at that which the poets teach,— he arrives at this by means of the Onkâra; the wise arrives at that which is at rest, free from decay, from death, from fear,— the Highest."