Midrasch zu Wajikra 15:17

4) (Vayikra 15:17) ("And every garment and all skin upon which there shall be semen shall be washed in water, and it shall be unclean until the evening.") "garment and skin": This tells me only of garment and skin. Whence do I derive that all other implements are like garment and skin in this regard? From "every garment and all skin." Or, if you wish, "garment and skin" is written in respect to sheretz (Vayikra 11:32), and "garment and skin" is written in respect to a dead body, and "garment and skin" is written in respect to semen. Just as "garment and skin" written in respect to sheretz and a dead body applies to all implements, so, "garment and skin" written in respect to semen applies to all implements.

5) "upon which there shall be semen": even upon part of it. "semen": Just as "semen" below (Vayikra 11 verse 18) is semen as at its inception, so "semen" here is "semen" as at its inception — to include vitiated semen. From here they ruled: A woman who discharged semen on the third day (after coitus) is tahor. These are the words of R. Eliezer b. Azaryah. R. Yishmael says: Sometimes (this involves the passage of) four (conjugal) times; sometimes, of five times; sometimes, of six times. R. Akiva says: It always involves the passage of five times.

6) And if part of the first time has passed, this is completed by part of the sixth time.