הַכֹּהֲנִּים הֻבְדְּלוּ מִכְּלַל הַלְוִיִּם לַעֲבוֹדַת הַקָּרְבָּנוֹת שֶׁנֶּאֱמַר (דברי הימים א כג יג) "וַיִּבָּדֵל אַהֲרֹן לְהַקְדִּישׁוֹ קֹדֶשׁ קָדָשִׁים". וּמִצְוַת עֲשֵׂה הִיא לְהַבְדִּיל הַכֹּהֲנִים וּלְקַדְּשָׁם וְלַהֲכִינָם לְקָרְבָּן שֶׁנֶּאֱמַר (ויקרא כא ח) "וְקִדַּשְׁתּוֹ כִּי אֶת לֶחֶם אֱלֹהֶיךָ הוּא מַקְרִיב":
The priests were set aside from the Levites as a whole [to perform] the sacrificial service, as [I Chronicles 23:13] states: "And Aaron was set apart to sanctify him as holy of holies." It is a positive commandment to distinguish the priests, make them holy, and prepare them for the sacrifices, as [Leviticus 21:8] states: "And you shall sanctify him because he offers the food of your G-d."
