Numbers 3:4-9

But Nadab and Abihu died in the LORD’s presence in the wilderness of Sinai when they burned before the LORD the wrong kind of fire, different than he had commanded. Since they had no sons, this left only Eleazar and Ithamar to serve as priests with their father, Aaron. Then the LORD said to Moses, “Call forward the tribe of Levi, and present them to Aaron the priest to serve as his assistants. They will serve Aaron and the whole community, performing their sacred duties in and around the Tabernacle. They will also maintain all the furnishings of the sacred tent, serving in the Tabernacle on behalf of all the Israelites. Assign the Levites to Aaron and his sons. They have been given from among all the people of Israel to serve as their assistants.