Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.3.51

9.2.3.51

yadevaitamatrāharanti | yānvai tāntsapta puruṣānekam puruṣamakurvannayameva sa yo 'yamagniścīyate 'tha yāmeṣāṃ tāmūrdhvāṃ śriyaṃ rasaṃ samudauhanneṣa sa yametamatrāgnimāharanti tadyadetamatrāharanti yaivaiteṣā saptānām puruṣāṇāṃ śrīryo rasastametadūrdhvaṃ samudūhanti tadasyaitacira ātmāyamagniścata ātmānamevāsyaitatsaṃskṛtya śiraḥ pratidadhāti

And, again, as to why they bring it here,--the Agni (fire-altar) which is built up here is the same as those seven persons which they made into one person; and that excellence and life-sap of them which they concentrated above, that is the fire which they now bring here,--hence, when they now bring it here, they concentrate above (in the head) that excellence and life-sap which belonged to those seven persons,--that is his (Agni's) head, and this built-up fire-altar is the body: having thus completed his body, he restores the head to it.