TT #207 · Parshas Tazria · Aliyah 5 · Passuk 33
English: he shall shave himself, but he shall not shave the netek; and the priest shall isolate the netek for a second seven-day period.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:33
Torah Temimah cites Sifra (Toras Kohanim) for preserving two hairs beside the netek so that later spread can be recognized.
Sifra — Vayikra 13, Midrash Sefer
English: One must not shave immediately beside the netek. Instead, shave outside it and leave two hairs next to it, so that any later spread can be recognized.
Textual derivation
Two hairs are left beside the netek so a later examination can tell whether its boundary has spread.