Nazir 40A

שאין דנין קל מחמור להחמיר עליו.

as one does not derive a halakha in a lenient case from the halakha in a more stringent one in a manner that would cause one to be stringent in the more lenient case. Since the case of a metzora is more stringent than that of a nazirite, as a metzora must shave his entire body, one cannot derive from the halakhot of a metzora that a stringent halakha applies to a nazirite. It is therefore necessary for the verse to specify that a nazirite’s final shaving must be performed with a razor.

רבי אומר: אינו צריך, הרי הוא אומר ״תער לא יעבור על ראשו עד מלאת״, התורה אמרה: ״אחר מלאת״⁠ — לא תהא תגלחת אלא בתער.

R' Yehuda HaNasi says that the term “razor” itself is not necessary to teach that a nazirite’s final shaving must be performed with a razor, as the verse states: “A razor should not come upon his head until the completion of the days that he vowed naziriteship to God” (Numbers 6:5). This full sentence indicates that the Torah said: After the completion of his term the shaving must be performed only with a razor.

והכתיב ״תער לא יעבור על ראשו״!

The Talmud comments: R' Yehuda HaNasi claims that the term “razor” indicates that shaving, in this case that of a nazirite after the completion of his term, may be performed only with a razor. If so, the verse should be understood in the same manner with regard to the prohibition of shaving. But if one examines the first part of the verse by itself, isn’t it written: “A razor should not come upon his head” (Numbers 6:5), and the first tanna, whose opinion R' Yehuda HaNasi does not dispute, derives from this verse that he is prohibited from shaving by other means as well, notwithstanding the explicit mention of a razor.