Numbers 3:11

8 And they will have the care of all the vessels of the Tent of meeting, and will do for the children of Israel all the necessary work of the House.

9 Give the Levites to Aaron and his sons; so that they may be his without question from among the children of Israel.

10 And give orders that Aaron and his sons are to keep their place as priests; any strange person who comes near is to be put to death.

12 See, I have taken the Levites out of the children of Israel to be mine in place of the first sons of the children of Israel;

13 For all the first sons are mine; on the day when I put to death all the first sons in the land of Egypt, I took for myself every first male birth of man and beast. They are mine; I am the Lord.

14 And the Lord said to Moses in the waste land of Sinai,