TT #108 · Parshas Tazria · Aliyah 2 · Passuk 10
English: The kohen shall look [at it]. And, behold! there is a white se'eith on the skin, and either it has turned the hair white, or there is healthy, live flesh in the se'eith.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:10
Torah Temimah cites Sifra (Toras Kohanim) for distinguishing a sign that turned white on its own from one changed by a neighboring lesion.
Sifra — Vayikra 13, Midrash Sefer
English: It turned by itself, not because another lesion turned it.
Textual derivation
The sign must arise independently; a change caused by an adjacent lesion does not meet this wording.