וְאִי כְּתַב רֹאשׁוֹ וְלָא כְּתַב זְקָנוֹ הֲוָה אָמֵינָא מַשְׁמַע תַּרְתֵּי דְּאָתֵי עֲשֵׂה וְדָחֵי אֶת לֹא תַעֲשֶׂה וּמַשְׁמַע דְּהַקָּפַת כׇּל הָרֹאשׁ שְׁמָהּ הַקָּפָה וְאַכַּתִּי בְּתַעַר מְנָלַן לְהָכִי כְּתַב רַחֲמָנָא זְקָנוֹ 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.
וְרַבִּי אֱלִיעֶזֶר דְּאָתֵי עֲשֵׂה וְדָחֵי אֶת לֹא תַעֲשֶׂה מְנָלֵיהּ יָלֵיף מִגְּדִילִים דְּתַנְיָא לֹא תִלְבַּשׁ שַׁעַטְנֵז 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),