TT #200 · Parshas Tazria · Aliyah 5 · Passuk 31
English: If the priest examines the netek and its appearance is not deeper than the skin, and there is no black hair in it, he shall isolate the netek for seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:31
Torah Temimah cites Sifra (Toras Kohanim) for requiring qualifying hair to be established within the netek.
Sifra — Vayikra 13, Midrash Sefer
English: “There is none in it” means the relevant hair must be firmly situated within the netek, not merely near its boundary.
Textual derivation
Only hair located within the netek counts for this rule. Hair merely touching or lying beside its boundary is not hair “in it.”