TT #103 · Parshas Tazria · Aliyah 2 · Passuk 7
English: However, if the mispachath spreads on the skin after it has been shown to the kohen for its purification, it shall be shown to the kohen a second time.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:7
Torah Temimah cites Sifra (Toras Kohanim) for the rule that spreading is assessed only after the initial examination.
Sifra — Vayikra 13, Midrash Sefer
English: Spreading does not render him unclean at the outset; it is considered after the lesion has been shown.
Textual derivation
Spreading is a later criterion, not an initial diagnosis: the lesion must first be presented and assessed before subsequent spread can determine the status.