Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.19

3.4.1.19

āsādya havīṃṣyagnim manthati | śiro vai yajñasyātithyaṃ janayanti vā enametadyanmanthanti śīrṣato vā agre jāyamāno jāyate śīrṣata evaitadagre yajñaṃ janayatyagnirvai sarvā devatā agnau hi sarvābhyo devatābhyo juhvati śiro vai yajñasyātithyaṃ śīrṣata evaitadyajñaṃ sarvābhirdevatābhiḥ samardhayati tasmādagnim manthati

When he has placed the sacrificial dishes (on the altar), he churns the fire. For the guest-offering is the head of the sacrifice; and in churning (the fire) they produce that (sacrifice); and one who is born is born with the head first: hence he thereby makes the sacrifice to be produced with the head first. Further, Agni means all the gods, since offering is made in the fire to all gods; and the guest-offering is the head of the sacrifice: hence, through all the deities, he secures success to the sacrifice from the very head (beginning). This is why he churns the fire.