Shatapatha Brahmana 10.4.1.8

10.4.1.8

atha yaścite'gnirnidhīyate tadekaṃ rūpamubhau bhavatastasmāttāvetenaiva rūpeṇemāḥ prajāḥ prajanayataḥ saiṣaikaiveṣṭakāgnireva tāmeṣa sarvo'gnirabhisampadyate saiveṣṭakāsampattadetadekamevākṣaraṃ vaugiti tadeṣa sarvo'gnirabhisampadyate saivākṣarasampat

And the two become that one form, to wit, the fire which is placed on the built (altar), and hence those two, by means of that form, produce creatures. Now Agni, indeed, is this single brick, and into this the whole Agni passes: this, indeed, is the perfection of bricks,--it is that one syllable (akshara) 'vauk,' it is this into which the whole Agni passes, and which is the perfection of syllables.