TT #164 · Parshas Tazria · Aliyah 3 · Passuk 22
English: If it spreads in the skin, the priest shall pronounce him unclean; it is an affliction.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:22
Torah Temimah cites Sifra (Toras Kohanim) for the requirement that the priest’s declaration of impurity be certain rather than doubtful.
Sifra — Vayikra 13, Midrash Sefer
English: 'The priest shall pronounce him unclean': he declares certain impurity, not a doubtful case.
Textual derivation
The priest declares a person impure only when the case is definite; uncertainty alone does not support this declaration.