Śatapatha Brāhmaṇa ("of a hundred paths") 13.5.2.2

13.5.2.2

niṣth\iteṣu pānnejaneṣu mahiṣīmaśvāyopanipādayantyathaināvadhivāsena samprorṇuvanti svargeloke prorṇuvathāmityeṣa vai svargo loko yatra paśuṃ sañjñapayanti nirāyatyāśvasya śiśnam mahiṣyupasthe nidhatte vṛṣā vājī retodhā reto dadhātviti mithunasyaiva sarvatvāya

When the foot-water is ready, they cause the Mahishî to lie down near the horse, and cover her up with the upper cloth, with 'In heaven ye envelop yourselves,'--for that indeed is heaven where they immolate the victim,..., 'May the vigorous male, the layer of seed, lay seed!' she says for the completeness of union.