Numbers 3:7

4 Nadab and Abihu died before Yahweh, when they offered strange fire before Yahweh, in the wilderness of Sinai, and they had no children; and Eleazar and Ithamar ministered in the priest`s office in the presence of Aaron their father.

5 Yahweh spoke to Moses, saying,

6 Bring the tribe of Levi near, and set them before Aaron the priest, that they may minister to him.

7 They shall keep his charge, and the charge of the whole congregation before the tent of meeting, to do the service of the tent.

8 They shall keep all the furniture of the tent of meeting, and the charge of the children of Israel, to do the service of the tent.

9 You shall give the Levites to Aaron and to his sons: they are wholly given to him on the behalf of the children of Israel.

10 You shall appoint Aaron and his sons, and they shall keep their priesthood: and the stranger who comes near shall be put to death.