Shatapatha Brahmana 10.1.4.13

10.1.4.13

tadāhuḥ na vayaso'gnicidaśnīyādvayo vā eṣa rūpam bhavati yo'gniṃ cinuta īśvara ārtimātostasmānna vayaso'gnicidaśnīyāditi tadvai kāmamevaivaṃvidaśnīyādagnervā eṣa rūpam bhavati yo'gniṃ cinute sarvaṃ vā idamagnerannaṃ sarvam ma idamannamityevaivaṃvidvidyāditi

Here, now, they say, 'He who has built an altar must not eat of any bird, for he who builds a fire-altar becomes of a bird's form; he would be apt to incur sickness: the Agnikit therefore must not eat of any bird.' Nevertheless, one who knows this may safely eat thereof; for he who builds an altar becomes of Agni's form, and, indeed, all food here belongs to Agni: whosoever knows this will know that all food belongs to him.