Rashi on Leviticus 13:33/רש"י על ויקרא י״ג:ל״ג

1והתגלח AND HE SHALL CUT HIS HAIR around the scall,

2ואת הנתק לא יגלח BUT THE SCALL ITSELF HE SHALL NOT CUT THE HAIR OF — this means that he leaves a row of two hairs quite close to it around it in order that it may become discernible if it (the scall) spreads, for if it spreads it passes the hair and will extend to the spot where the hair has been cut off (Mishnah Negaim 10:5; Sifra, Tazria Parashat Nega'im, Chapter 9 7).