TT #153 · Parshas Tazria · Aliyah 3 · Passuk 19
English: If, at the place of the boil, there is a white swelling or a bright reddish-white spot, it shall be shown to the priest.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:19
Sifra (Toras Kohanim).
Sifra — Vayikra 13, Midrash Sefer
English: The boil must precede the swelling; the swelling must arise at the site of an earlier boil.
Textual derivation
The sequence in the verse defines this as a lesion arising at the site of an earlier boil.