TT #167 · Parshas Tazria · Aliyah 3 · Passuk 23
English: If the bright spot remains in its place and has not spread, it is the scar of the boil, and the priest shall pronounce him clean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:23
Torah Temimah cites Sifra (Toras Kohanim) for the requirement that the former boil site remain visibly identifiable.
Sifra — Vayikra 13, Midrash Sefer
English: The scarred area must be visibly burned/scarred and its location must remain recognizable.
Textual derivation
The scarred area must both have a burned/scarred character and remain recognizable as the original site.