TT #303 · 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 Chullin 31b for comparing the second washing with the first.
E-Daf — Chullin 31b · Shas.org · Sefaria — Chullin 31b, source text and translation
English: Rabbi Yonatan ben Yosef compares the second washing with the first.
Textual derivation
The rules governing the second washing are derived by comparing it with the first washing.