Reference for Zevachim 31:3

טמא יוכיח מה לטמא שכן מטמא

As for an unclean [priest], the reason is that he defiles [th flesh of the sacrifice]! Then let one with a blemish prove it. And thus the argument revolves, the distinguishing feature of one not being that of the other, and the distinguishing feature of the other not being that of the first.

Jastrow