TT #187 · Parshas Tazria · Aliyah 5 · Passuk 29
English: If a man or woman has an affliction on the head or on the beard,
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:29
Torah Temimah cites Sifra (Toras Kohanim) for the rule that a netek does not spread from the head to the beard or vice versa.
Sifra — Vayikra 13, Midrash Sefer
English: Even though head and beard do not combine, could spreading pass from one to the other? Scripture's separate designation excludes that as well.
Textual derivation
Even though the head and beard are distinct areas, the rule also bars a netek from spreading from one area into the other.