TT #279 · Parshas Tazria · Aliyah 7 · Passuk 51
English: The priest shall examine the lesion on the seventh day; if it has spread in the garment, warp or woof, or leather article, the lesion is malignant tzaraas; it is impure.
Torah Temimah cites Sifra (Toras Kohanim) for the prohibition on benefit from a garment definitively declared impure.
Sifra — Vayikra 13, Midrash Sefer
English: “Malignant tzaraas” teaches that a garment declared definitively impure is cursed and may not be used for benefit.
Textual derivation
Once the priest has definitively declared the garment impure, it is prohibited to derive benefit from it; the Sifra next asks whether the same applies while it is only under quarantine.