Numbers 3:4-9

But Nadab and Abihu died in the presence of the LORD when they offered the wrong kind of fire before the LORD in the Desert of Sinai. They had no sons. So Eleazar and Ithamar served as priests during the lifetime of their father Aaron. The LORD said to Moses, “Bring the tribe of Levi and present them to Aaron the priest to help him. They will help him and all the Israelites at the Meeting Tent, doing the work in the Holy Tent. The Levites must take care of everything in the Meeting Tent and serve the people of Israel by doing the work in the Holy Tent. Give the Levites to Aaron and his sons; of all the Israelites, the Levites are given completely to him.