Numbers 8:15

8:15 “After this, the Levites will go in to do the work of the tent of meeting. So you must cleanse them and offer them like a wave offering.

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.”