Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.2.4

7.4.2.4

sa yaḥ sa prajāpatirvyasraṃsata | ayameva sa yo 'yamagniścīyate 'tha yā sā pratiṣṭhaiṣā sā prathamā svayamātṛṇā tadyadetāmatropadadhāti yadevāsyaiṣātmanastadasminnetatpratidadhāti tasmādetāmatropadadhāti

Now that Pragâpati who became disjointed is this very Agni (fire-altar) that is now being built up; and that resting-place (or, foundation) is this first naturally-perforated (brick);--thus when he now puts it on, he thereby puts upon this (altar-site) that (foundation) which there was for his body: that is why he now puts it on.