Shatapatha Brahmana 10.1.1.10

10.1.1.10

eṣo atrāpītiḥ na ha vā asyāputratāyai kā cana śaṅkā bhavati ya evametau mithunāvātmānaṃ cāgniṃ ca vedānnaṃ ha tvevāyamātmā dakṣiṇānnaṃ vanute yo na ātmeti hyapyṛṣiṇābhyuktam

This also is the entering therein;--there is indeed no fear of him (Agni) being without offspring to whosoever thus knows these two, the body and Agni, to be a pair; but, indeed, this body is food, as is said by the Rishi (Rig-veda X, 107, 7), 'The Dakshinâ winneth food which is our own self (breath).'