Num 18:7

וְאַתָּ֣ה וּבָנֶ֣יךָ אִ֠תְּךָ תִּשְׁמְר֨וּ אֶת־כְּהֻנַּתְכֶ֜ם לְכָל־דְּבַ֧ר הַמִּזְבֵּ֛חַ וּלְמִבֵּ֥ית לַפָּרֹ֖כֶת וַעֲבַדְתֶּ֑ם עֲבֹדַ֣ת מַתָּנָ֗ה אֶתֵּן֙ אֶת־כְּהֻנַּתְכֶ֔ם וְהַזָּ֥ר הַקָּרֵ֖ב יוּמָֽת׃ ס

But only you and your sons shall attend to your priesthood for everything concerning the altar and what is inside the veil, and you are to perform that service. I am giving you the work of the priesthood as a gift, but any outsider who comes near the sanctuary must be put to death.”