Numbers 18:5-7

5You must take care of God's Holy Place and the altar. Then I will not be angry with the Israelites again.

6I have chosen your family, the Levites, from among the other Israelite tribes. They are the Lord's gift to you, to help you take care of the Tent of Meeting.

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.’