Numbers 3:6-9

3:6 “Bring the tribe of Levi near, and present them before Aaron the priest, that they may serve him. 3:7 They are responsible for his needs and the needs of the whole community before the tent of meeting, by attending to the service of the tabernacle. 3:8 And they are responsible for all the furnishings of the tent of meeting, and for the needs of the Israelites, as they serve in the tabernacle. 3:9 You are to assign the Levites to Aaron and his sons; they will be assigned exclusively to him out of all the Israelites.

Numbers 8:19

8:19 I have given the Levites as a gift to Aaron and his sons from among the Israelites, to do the work for the Israelites in the tent of meeting, and to make atonement for the Israelites, so there will be no plague among the Israelites when the Israelites come near the sanctuary.”

Numbers 8:22

8:22 After this, the Levites went in to do their work in the tent of meeting before Aaron and before his sons. As the Lord had commanded Moses concerning the Levites, so they did.