Numbers 3:4-9

But Nadab and Abihu died before the LORD when they offered strange fire before the LORD in the wilderness of Sinai; and they had no children. So Eleazar and Ithamar served as priests in the lifetime of their father Aaron. Then the LORD spoke to Moses, saying, “Bring the tribe of Levi near and set them before Aaron the priest, that they may serve him. They shall perform the duties for him and for the whole congregation before the tent of meeting, to do the service of the tabernacle. They shall also keep all the furnishings of the tent of meeting, along with the duties of the sons of Israel, to do the service of the tabernacle. You shall thus give the Levites to Aaron and to his sons; they are wholly given to him from among the sons of Israel.