Maitri (Maitrāyaṇīya) Upaniṣad 7.11

7.11

8 triṣvekapāccaredbrahma tripāccarati cottare |
satyānṛtopabhogārthāḥ dvaitībhāvo mahātmana iti dvaitībhāvo mahātmana iti ||

This is indeed the nature of it (the Veda), the supreme light of the ether which is within the heart. This is taught as threefold, in the fire, in the sun, in the breath. This is indeed the nature of it, the syllable Om, of the ether which is within the heart. By it (by the Om) that (light) starts, rises, breathes forth, becomes for ever the means of the worship and knowledge of Brahman. That (light, in the shape of Om), when there is breathing, takes the place of the internal heat, free from all brightness. This is like the action of smoke; for when there is a breath of air, the smoke, first rising to the sky in one column, follows afterwards every bough, envelopes it and takes its shape. It is like throwing salt (into water), like heating ghee. The Veda comes and goes like the dissolving view of a master-magician. And here they quote:
'Why then is it called "like lightning?" Because as soon as it comes forth (as Om) it lights up the whole body. Therefore let a man worship that boundless light by the syllable Om.'
(1) The man in the eye who abides in the right eye, he is Indra, and his wife abides in the left eye.
(2) The union of these two takes place in the cavity within the heart, and the ball of blood which is there, that is indeed the vigour and life of these two.
(3) There is a channel going from the heart so far, and fixed in that eye; that is the artery for both of them, being one, divided into two.
(4) The mind excites the fire of the body, that fire stirs the breath, and the breath, moving in the chest, produces the low sound.
(5) Brought forth by the touch of the fire, as with a churning-stick, it is at first a minim, from the minim it becomes in the throat a double minim; on the tip of the tongue know that it is a treble minim, and, when uttered, they call it the alphabet (στοιχεῖα).
(6) He who sees this, does not see death, nor disease, nor misery, for seeing he sees all (objectively, not as affecting him subjectively); he becomes all everywhere (he becomes Brahman).
(7) There is the person in the eye, there is he who walks as in sleep, he who is sound asleep, and he who is above the sleeper: these are the four conditions (of the Self), and the fourth is greater than all.
(8) Brahman with one foot moves in the three, and Brahman with three feet is in the last.
It is that both the true (in the fourth condition) and the untrue (in the three conditions) may have their desert, that the Great Self (seems to) become two, yes, that he (seems to) become two.