Shatapatha Brahmana 1.8.2.9

1.8.2.9

atha yadanuyājānyajati | candāṃsi vā anuyājāścandāṃsyevaitatsaṃtarpayati tasmādanuyājānyajati tasmādyena vāhanena dhāvayettadvimucya brūyātpāyayatainatsuhitaṃ kurutetyeṣa u vāhanasyāpahnavaḥ

And this again is why he makes the after-offerings. The after-offerings are the metres: hence he thereby gratifies the metres, and for this reason also he makes the after-offerings. By whatever team, therefore, he has himself drawn, that (team) he would thereby unyoke, saying, 'Give it to drink, feed it well!' and thus his team is propitiated.