TT #158 · Parshas Tazria · Aliyah 3 · Passuk 20
English: The priest shall examine it. If its appearance is lower than the skin and its hair has turned white, the priest shall pronounce it unclean; it is a tzaraas affliction that has broken out in the boil.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:20
Sifra (Toras Kohanim).
Sifra — Vayikra 13, Midrash Sefer
English: The lesion has broken out in the boil itself, not merely in ordinary skin or in burn skin.
Textual derivation
The special rule is tied specifically to a lesion arising in the earlier boil.