TT #309 · Parshas Tazria · Aliyah 7 · Passuk 59
English: This is the law of the lesion of tzaraas in a garment of wool or linen, in warp or woof, or in any leather article, to declare it clean or impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:59
Torah Temimah cites Sifra (Toras Kohanim) for the duty to state the correct ruling whether it establishes purity or impurity.
Sifra — Vayikra 13, Midrash Sefer
English: The phrase teaches that just as it is a mitzvah to declare an article pure, it is also a mitzvah to declare it impure when required.
Textual derivation
The priest has a duty to issue the ruling the evidence requires in either direction: declaring an article pure when appropriate and impure when appropriate.