TT #209 · 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.
Sifra (Toras Kohanim).
English: The netek itself is not shaved, and two hairs are left beside it.
Textual derivation
The procedure preserves the boundary needed for subsequent examination.