TT #165 · 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 limiting the relevant spread to the area directly beneath the lesion.
Sifra — Vayikra 13, Midrash Sefer
English: It spreads in the area directly beneath it, not into ordinary skin or burn skin.
Textual derivation
Spread into ordinary skin or burn skin does not count here; the rule concerns the area directly beneath the original lesion.