Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.16

1.5.3.16

atha caturthe prayāje samānayati barhiṣi | prajā vai brahīreta ājyaṃ tatprajāsvevaitadretaḥ sicyate tena retasā siktenemāḥ prajāḥ punarabhyāvartam prajāyante tasmāccaturtheprayāje samānayati barhiṣi

Now at the fourth fore-offering, to the barhis, he pours (butter) together (into the guhû). The barhis, namely, represents descendants, and the butter seed: hence seed is thereby infused into the descendants, and by that infused seed descendants are generated again and again. For this reason he pours together (butter) at the fourth fore-offering, that to the barhis.