Nazir 58B

דלא אתי עשה ודחי את לא תעשה ועשה, לילף מכהן, דדחי!

that a positive mitzva does not come and override a prohibition and a positive mitzva, let him derive from the fact that a leprous priest must shave that this mitzva does override a prohibition and another positive mitzva.

אלא, מכהן לא ילפינן: מה לכהן שכן לאו שאינו שוה בכל. נזיר מכהן נמי לא יליף, שכן לאו שאינו שוה בכל.

Rather, he would explain that we do not derive other cases of the Torah from the halakha of a priest, for the following reason: What is different about the prohibition of a priest is that it is a prohibition that is not equally applicable to all, i.e., the prohibitions of a priest do not apply to Israelites. For the same reason, the case of a nazirite also cannot be derived from that of a priest, as the prohibition of a priest is a prohibition that is not equally applicable to all, in contrast to that of a nazirite. Consequently, the derivation from “his head” is necessary to teach that the shaving of a metzora overrides the prohibition stated with regard to a nazirite.

ומאן דמוקים להאי ״ראשו״ בנזיר, למה לי ״זקנו״?

The Talmud turns its attention to the other opinion: And according to the one who establishes this verse: “He should shave all his hair; his head and his beard” (Leviticus 14:9), as referring to a nazirite, i.e., he derives from this verse the principle that in certain cases, including that of a leprous priest, a positive mitzva overrides a prohibition and a positive mitzva, why do I need the term “his beard”?

מיבעי ליה, לכדתניא: ״זקנו״ מה תלמוד לומר? לפי שנאמר ״ופאת זקנם לא יגלחו״, יכול אף כהן מצורע כן, תלמוד לומר: ״זקנו״.

The Talmud answers: He requires it for that which is taught in a baraita with regard to the shaving of a metzora, which must be performed with a razor: Why must the verse state: “His beard,” as it already stated that he shaves all his hair? Since it is stated: “Neither should they shave off the corners of their beard” (Leviticus 21:5), one might have thought that even a priest who is a metzora is included in the prohibition against shaving. The verse therefore states: “His beard,” to teach that a leprous priest must shave his beard as well.