Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.3.16

3.5.3.16

atha yatpatnyakṣasya saṃtāpamupānakti | prajananamevaitatkriyate yadā vai striyai ca puṃsaśca saṃtapyate 'tha retaḥ sicyate tattataḥ prajāyate parāgupānakti parāgghyeva retaḥ sicyate 'thāha havirdhānābhyām pravartyamānābhyāmanubrūhīti

And in that the wife anoints the burning (part) of the axle, thereby a productive union is effected; for when woman and man become heated, the seed flows, and thereupon birth takes place. She anoints in a direction away (from the cart), for away the seed is cast. He then says (to the Hotri), 'Recite to the Soma-carts as they are wheeled forward!'