Parashas Metzora · Passuk 14:35
Chabad.org — English translation + Rashi
Torah Temima points to Negaim 12:5.
English: “Something like a nega” — “like a nega” and not “a nega.” Even a Torah scholar who knows with certainty that it is a nega must not decide and say “a nega,” but only “something like a nega.”
Owner’s language
Chazal forbid the owner (even a scholar) from pronouncing the definitive diagnosis; only the kohen may declare it a nega.