Numbers 8:15, 19

“After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting.

From among all the Israelites, I have given the Levites as gifts to Aaron and his sons to do the work at the tent of meeting on behalf of the Israelites and to make atonement for them so that no plague will strike the Israelites when they go near the sanctuary.”