Numbers 18:7-8

7But only you, Aaron, and your sons will serve me as priests. You will work at the altar and in the Holy Place behind the curtain. I have chosen you to serve me as my priests. That is my gift to you. Any other person who comes near the Holy Place must die.’

8The Lord said to Aaron, ‘I give you authority over the things that people offer to me. As priests, part of the gifts that people bring will belong to you and your sons. This rule will always be true for you and your descendants.