Shatapatha Brahmana 10.1.1.7

10.1.1.7

sa eṣa mithuno'gniḥ prathamā ca citirdvitīyā ca tṛtīyā ca caturthī cātha pañcamyai citeryaścite'gnirnidhīyate tanmithunam mithuna u evāyamātmā

Now, that Agni (fire-altar) consists of pairs--the first layer and the second, and the third and fourth; and of the fifth layer the fire which is placed on the built (altar) is the mate. And, indeed, this body consists of pairs.