Śatapatha Brāhmaṇa ("of a hundred paths") 7.2.2.2

7.2.2.2

pratyetya prāyaṇīyena pracarati | prāyaṇīyena pracarya sīraṃ yunaktyetadvā enaṃ devāḥ saṃskariṣyantaḥ purastādannena samārdhayaṃstathaivainamayametatsaṃskariṣyanpurastādannena samardhayati sīram bhavati seraṃ haitadyatsīramirāmevāsminnetaddadhāti

Having returned (to the offering, or hall-door fire) he proceeds with the opening sacrifice. Having performed the opening sacrifice, he yokes a plough. For the gods at that time, being about to heal him (Agni-Pragâpati), first supplied him with food, and in like manner does this (Sacrificer) now that he is about to heal him, first supply him with food. It (the food) is the plough (sîra), for 'sîra' is the same as 'sera:' he thus puts food into him.