Midrasch zu Wajikra 15:3

1) "And this shall be his tumah in his flux": His tumah is contingent upon his flux (i.e., sightings) and not upon (the passage of) days (— two sightings for tumah, three for an offering.)

2) (For without this verse we would say:) Now if a zavah, who confers tumah upon one who lives with her, (as a niddah does,) requires (stringent) tumah, three sightings on three days, then a zav, who does not confer tumah upon one he lives with, how much more so should he require for (stringent) tumah three sightings on three days. It is, therefore, written "And this shall be his tumah in his flux." His tumah is contingent upon his flux (i.e., sightings) and not upon (the passage of) days.

3) (Vayikra 15:3) "in his flux … his flux … with his flux": We are hereby taught that he acquires (stringent) tumah with three sightings. This ("flux") tells me only of long (flows). Whence do I derive (the same for) short ones? From "shall be" — any amount. "his flesh drips" — any amount. "his flesh be stopped up" — any amount. "his flesh" — any amount. If short ones are mentioned, why mention long ones? To set a (time) limit for long ones. So that if he sees one (flux) as (long as) three (short ones) — (the time it takes to walk) from Gidiyon to Shiloach, corresponding to two immersings and two wipings — he is a confirmed zav.