Vayikra 13:54

את אשר בו הנגע THAT WHEREIN THE PLAGUE IS — One might think that he has to wash only that spot in which is the plague and not the garment itself! It. however, states: Not, “he shall wash the plague, but “that where in the plague is”. If this be so, then one might think that the entire garment requires washing! It, however, states, (v. 55) הנגע “the plague” (after the plague has been washed), and not the garment. How shall it be done? He washes some of the garment with it (the plague) (Sifra, Tazria Parashat Nega'im, Chapter 15 5).

וְצִוָּה֙ הַכֹּהֵ֔ן וְכִ֨בְּס֔וּ אֵ֥ת אֲשֶׁר־בּ֖וֹ הַנָּ֑גַע וְהִסְגִּיר֥וֹ שִׁבְעַת־יָמִ֖ים שֵׁנִֽית׃

then the priest shall command that they wash the thing in which the plague is, and he shall shut it up seven days more: