Maitri (Maitrāyaṇīya) Upaniṣad 6.23

23. Now, it has elsewhere been said: ‘The sound-Brahma is the syllable *Om*. That which is its acme is tranquil, soundless, fearless, sorrowless, blissful, satisfied, steadfast, immovable, immortal, unshaken, enduring, named Vishṇu (the Pervader). So for paramountcy one should reverence both these. For thus has it been said:—

Who is both higher and lower,
That god, known by the name of *Om*,
Soundless and void of being, too—
Thereon concentrate in the head!

24. Now, it has elsewhere been said: ‘The body is a bow. The arrow is *Om*. The mind is its point. Darkness is the mark. Having pierced through the darkness, one goes to what is not enveloped in darkness. Then, having pierced through what is thus enveloped, one sees Him who sparkles like a wheel of fire, of the color of the sun, mightful, the Brahma that is beyond darkness, that shines in yonder sun, also in the moon, in fire, in lightning. Now, assuredly, when one has seen Him, one goes to immortality.’ For thus has it been said:—

The meditation that is on the highest principle within
Is also directed upon outer objects.
Hence the unqualified understanding
Comes into qualifiedness.