Shatapatha Brahmana 10.4.1.1

10.4.1.1

prajāpatiṃ visrastam yatra devāḥ samaskurvaṃstamukhāyāṃ yonau reto bhūtamasiñcanyonirvā ukhā tasmā etatsaṃvatsare'nnaṃ samaskurvanyo'yamagniścitastadātmanā paryadadhustadātmanā parihitamātmaivābhavattasmādannamātmanā parihitamātmaiva bhavati

When the gods restored the relaxed Pragâpati, they poured him, as seed, into the fire-pan (ukhâ) as the womb, for the fire-pan is a womb. In the course of a year they prepared for him this food, to wit, the fire-altar built here, and enclosed it in a body; and, being enclosed in a body, it became the body itself; whence food, when enclosed in a body, becomes the body itself.