Shatapatha Brahmana 10.1.1.9

10.1.1.9

eṣātrāpītiḥ apyahaivam mithuna itthaṃ ha tvevāpi mithuno vāgeveyaṃ yo'yamagniścito vācā hi cīyate'tha yaścite'gnirnidhīyate sa prāṇaḥ prāṇo vai vāco vṛṣā prāṇo mithunaṃ vāgvevāyamātmātha ya ātmanprāṇastanmithunaṃ so'nena mithunenātmanaitam mithunamagnimapyeti

This, then, is the entering therein;--even thus, indeed, he (Agni) consists of pairs; but in this way also he consists of pairs:--the fire-altar here built up is no other than this speech, for with speech it is built up; and the fire which is placed on the built (altar) is the breath; and the breath ('prâna,' m.) is the male, the mate, of speech ('vâk,' f.). And, indeed, this body is speech; and the breath which is in the body is its mate: with this mated body that (vital fluid) thus enters into the mated Agni.