Numbers 3:4-9

But Nadab and Abihu died in the presence of the LORD when they offered strange (unholy, unacceptable, inappropriate) fire before the LORD in the Wilderness of Sinai; and they had no sons. So Eleazar and Ithamar served as priests in the presence and under the supervision of Aaron their father. [Lev 10:1-4] Then the LORD spoke to Moses, saying, “Bring the tribe of Levi near and present them before Aaron the priest, so that they may serve him. They shall carry out the duties for him and for the whole congregation before the Tent of Meeting, doing the service of the tabernacle. They shall also take care of all the furnishings and utensils of the Tent of Meeting, and [attend to] the duties of the Israelites, doing the service of the tabernacle. You shall give the Levites to Aaron and to his sons [as servants and helpers]; they are to be given wholly to him from among the Israelites.