Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.4.23

2.4.4.23

sa vai paścādiva yajñasya juhoti | paścādvai parītya vṛṣā yoṣāmadhidravati tasyāṃ retaḥ siñcati sa vai prāgevāgre juhotyagne vīhātyanuvaṣaṭkaroti tatsviṣṭakṛdbhājanaṃ sa vai prāgeva juhoti

He offers, as it were, behind the sacrifice: for it is from behind that the male approaches and impregnates the female. He first offers in the east. With 'O Agni, accept...!' he repeats the Vashat,--this is in lieu of the Svishtakrit; and (the latter) he offers in the east.