Shatapatha Brahmana 9.5.1.55

9.5.1.55

yadvevaitayā maitrāvaruṇyā payasyayā yajate | prajāpatervisrastādretaḥ parāpatattaṃ yatra devāḥ samaskurvaṃstadasminnetayā maitrāvaruṇyā payasyayā reto 'dadhustathaivāsminnayametaddadhāti

And, again, as to why he offers this dish of clotted curds to Mitra and Varuna. When Pragâpati was released, the seed fell from him. When the gods restored him, they, by means of this dish of clotted curds, put seed into him; and in like manner does this (Sacrificer) thereby put seed into him.