Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.2.7

7.5.2.7

tadetadannaṃ sṛṣṭvā | purastātpratyagātmannadhatta tasmādyaḥ kaścānnaṃ sṛjate purastādevainatpratyagātmandhatte tadvā ukhāyāmudaraṃ vā ukhodare tadannaṃ dadhāti

Having created that food, he took it into himself from the front towards the back; and hence whosoever prepares for himself food, takes it into himself from the front towards the back (inside). That (animal food being put) in the fire-pan, and the fire-pan being the belly, he thus puts the food into the belly.