TT #160 · Parshas Tazria · Aliyah 3 · Passuk 21
English: If the priest examines it and there is no white hair in it, and it is not lower than the skin but is faded, he shall isolate the person for seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:21
Torah Temimah cites Sifra (Toras Kohanim) for applying the absence-of-hair test to the lesion and any hair extending from it.
Sifra — Vayikra 13, Midrash Sefer
English: 'No hair in it' means neither in the lesion itself nor in a hair extending from it.
Textual derivation
A hair extending from the lesion is included in the assessment; the phrase “no hair in it” is not limited to the skin patch’s interior.