Shatapatha Brahmana 9.2.1.22

9.2.1.22

yadvevāgnā utsādayati | śira etadyajñasya yatpravargya ātmāyamagniścitaḥ sa yadanyatrāgnerutsādayedbahirdhāsmāciro dadhyādatha yadagnā utsādayatyātmānamevāsyaitatsaṃskṛtya śiraḥ pratidadhāti

And, again, as to why he sets it out on the fire-altar,--the Pravargya is the head of the sacrifice, and this built-up fire-altar is the body: hence were he to set it out in any other place than the fire-altar, he would place that head away from that (body), but in that he sets it out on the fire-altar, he, having put together that body of him (Agni), restores the head to it.