Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.22

4.2.4.22

atha yadadhvaryuśca pratiprasthātā ca | niśca krāmataḥ pra ca padyete yathā baddhavatsopācaredevametaṃ grahamupācaratastamavanayati gāyatrīmevaitatprasrāvayati pratteyaṃ gāyatrī yajamānāya sarvānkāmāndohātā iti tasmādvā avanayati

And when both the Adhvaryu and the Pratiprasthâtri walk out (of the cart-shed) and (afterwards) enter (again), it is as if (a cow) were to come with the calf tied to her. They come to this cup of Soma, and he (the Adhvaryu) pours it out; whereby he lets loose the Gâyatrî: 'Made over to the sacrificer, may this Gâyatrî yield all his desires!' for this reason he pours it out.