TT #168 · Parshas Tazria · Aliyah 3 · Passuk 23
English: If the bright spot remains in its place and has not spread, it is the scar of the boil, and the priest shall pronounce him clean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:23
Torah Temimah cites Sifra (Toras Kohanim) for the requirement that a declaration of purity be based on a determinate case.
Sifra — Vayikra 13, Midrash Sefer
English: 'He shall pronounce him clean': he declares the certain case clean, not a doubtful case.
Textual derivation
The priest may pronounce a person clean when the facts establish that status; a doubtful case is not declared clean by this rule.