Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.2.4

4.3.2.4

tadupapalyayya pratigṛṇāti | idamevaitadretaḥ siktamupanimadatyatha yatparāṅ tiṣṭhanpratigṛṇīyātparāgu haivaitadretaḥ siktam praṇaśyettanna prajāyeta samyañcā u caivaitadbhūtvaitadretaḥ siktam prajanayataḥ

Having turned round (so as to face the Hotri, the Adhvaryu) then responds: thereby he quickens that implanted seed. On the other hand, were he to respond while standing with his face turned away (from the Hotri), that implanted seed would assuredly perish away, and would not be brought forth; but thus facing each other (the male and female) bring forth the implanted seed.