Numbers 3:7

4 And Nadab and Abihu were put to death before the Lord when they made an offering of strange fire before the Lord, in the waste land of Sinai, and they had no children: and Eleazar and Ithamar did the work of priests before Aaron their father.

5 And the Lord said to Moses,

6 Make the tribe of Levi come near, and put them before Aaron the priest, to be his helpers,

7 In order that they may be responsible to him and to all Israel for the care of the Tent of meeting, and to do the work of the House;

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.