Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.3.4

3.4.3.4

te 'gninaiva tvaca vipalyāṅgayanta | agnirvai mithunasya kartā prajanayitā tatprajāmupāyantsaṃtarāmaṅgulīrāñcanta saṃtarām mekhalāṃ tatprajāmātmannakurvata tatho evaiṣa etatprajāmevopaiti

By means of Agni (fire) they enveloped (the body) with a skin. Now, Agni being the causer of sexual union, the progenitor, they thereby obtained offspring. They turned in their fingers more tightly and drew their zone tighter, whereby they produced offspring for themselves. And in like manner does he (the sacrificer) thereby obtain offspring.