Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.1.25

1.3.1.25

athājyamavekṣate | taddhaike yajamānamavakhyāpayanti tadu hovāca yājñavaklyaḥ kathaṃ nu na svayamadhvaryavo bhavanti kathaṃ svayaṃ nānvāhuryatra bhūyasya ivāśiṣaḥ kriyante kathaṃ nveṣāmatraiva śraddhā bhavatīti yāṃ vai kāṃ ca yajña ṛtvija āśiṣamāśāsate yajamānasyaiva sā tasmādadhvaryurevāvekṣeta

The reason why he purifies the sprinkling water with the strainers covered with butter is, that he thereby puts milk into the water, and that the milk thereby (becomes) beneficial in the water, for, when it rains, plants are thereby produced; and on eating the plants and drinking the water, vital fluid (serum) results therefrom: and thus (he does this) in order to supply the vital fluid (of the sacrificer).