Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.18

1.5.3.18

yajamāna eva juhūmanu | yo smā arātīyati sa upabhṛtamanuyajamānāyaivaitaddviṣantam bhrātṛvyam baliṃ hārayatyattaiva juhūmanvādya upabhṛtamanvattra evaitadādyam baliṃ hārayati tasmāccaturthe prayāje samānayati

The sacrificer, doubtless, (stands) behind the guhû, and he who means evil to him, (stands) behind the upabhrit: hence he thereby makes the spiteful enemy pay tribute to the sacrificer. The consumer, doubtless, (stands) behind the guhû, and the one to be consumed behind the upabhrit hence he thereby makes the one that is to be consumed pay tribute to the consumer. This is the reason why he pours (butter) together at the fourth fore-offering, that to the barhis.