Shatapatha Brahmana 10.4.1.2

10.4.1.2

tathaivaitadyajamānaḥ ātmānamukhāyāṃ yonau reto bhūtaṃ siñcati yonirvā ukhā tasmā etatsaṃvatsare'nnaṃ saṃskaroti yo'yamagniścitastadātmanā paridadhāti tadātmanā parihitamātmaiva bhavati tasmādannamātmanā parihitamātmaiva bhavati

In like manner does the Sacrificer now pour his own self (or body), as seed, into the fire-pan as the womb, for the fire-pan is a womb. In the course of a year he prepares for it (his self) this food, to wit, the fire-altar here built, and encloses it in a body, and, being enclosed in a body, it becomes the body itself; whence food, when enclosed in a body, becomes the body itself.