Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.1.19

2.5.1.19

āsādya havīṃṣyagnim manthanti | agniṃ ha vai jāyamānamanu prajāpateḥ prajā jajñire tatho evaitasyāgnimeva jāyamānamanu prajā jāyante tasmādāsādya havīṃṣyagnim manthanti

On putting the, sacrificial dishes in their place, they churn the fire. For it was after Agni was born that Pragâpati's offspring was born;. and so for this (sacrificer) also offspring is born after Agni (the fire) has been produced: this is why they churn the fire, after they have deposited the sacrificial dishes in their place.