Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.3.5

3.4.3.5

so 'gninaiva tvacaṃ vipalyaṅgayate | agnirvai mithunasya kartā prajanayitā tatprajāmupaiti saṃtarāmaṅgulīracate saṃtarām mekhalāṃ tatprajāmātmankurute

By means of Agni he envelops himself with a skin. Now, Agni being the causer of sexual union, the progenitor, he thereby obtains offspring. He turns in his fingers more tightly and draws his zone tighter, whereby he produces offspring for himself.