מדרש אויף ויקרא 15:32

8) (Vayikra 15:32) ("This is the law of the zav and of him from whom semen issues forth to be made unclean thereby.") "This is the law of the zav"; of one sighting, who is comparable to one from whom semen issues forth (For a zav of two sightings must observe seven clean days); and he immerses and eats his Paschal offering at night.