Numbers 3:4-8

But Nadab and Abihu died in the presence of 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 forward and present them before Aaron the priest, that they may serve him.

They shall perform the duties for him and for the whole congregation in front of the tent of meeting, to do the service of the tabernacle.

They shall also take care of all the furnishings of the tent of meeting, along with the duties of the sons of Israel, to do the service of the tabernacle.