נזיר מ״א ב

And had the Torah written only: “His head,” and not written: “His beard,” I would say that “his head” teaches two matters. First, that a positive mitzva comes and overrides a prohibition. And it also teaches that rounding the entire head is called rounding. And still, from where do we derive that a leper must shave with a razor? For this reason the Merciful One also wrote: “His beard,” and we derive from the prohibition that bars priests from destroying their beards that the removal of the beard involves the use of a razor. The Gemara asks: And Rabbi Eliezer, from where does he derive the general principle that a positive mitzva will come and override a prohibition? The Gemara answers: He derives it from the mitzva of ritual fringes. As it is taught in a baraita: This verse: “You shall not wear diverse kinds of wool and linen” (Deuteronomy 22:11),