Shatapatha Brahmana 11.1.6.31

11.1.6.31

trīṇi śiśnāni ta eva trayo'nuyājāḥ sa yo'yaṃ varṣiṣṭho'nuyājastadidaṃ varṣiṣṭhamiva śiśnaṃ taṃ vā anavānanyajedityāhustatho hāsyaitadamṛdhram bhavatīti

The three after-offerings are the three male organs; and that which is the chief after-offering is, as it were, the chief organ. 'He should offer it without drawing breath,' they say, 'for thus it becomes unfailing for him.'