Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.1.11

3.3.1.11

tāṃ neṣṭā vācayati | toto rāya ityathaināṃ somakrayaṇyā saṃkhyāpayati vṛṣā vai somo yoṣā patnyeṣa vā atra somo bhavati yatsomakrayaṇī mithunamevaitatprajananaṃ kriyate tasmādenāṃ somakrayaṇyā saṃkhyāpayati

The Neshtri makes her say, 'Thine, thine is wealth;' whereupon he causes her to be looked at by the Soma-cow. Now, Soma is a male, and the wife is a female, and that Soma-cow becomes here (exchanged for) Soma: a productive union is thus effected;--this is why he causes her to be looked at by the Soma-cow.