TT #301 · Parshas Tazria · Aliyah 7 · Passuk 58
English: The garment, warp or woof, or any leather article that you wash, if the lesion disappears from it, shall be washed a second time and be clean.
Torah Temimah cites Zevachim 94a for the rule that washing leather requires rubbing.
E-Daf — Zevachim 94a · Shas.org · Sefaria — Zevachim 94a, source text and translation
English: Rava asks whether leather can be washed, since Scripture says “every leather article that you shall wash.” He concludes that washing without rubbing is not considered washing.
Textual derivation
For leather, the required washing must involve the kind of rubbing that qualifies as washing.