Numbers 8:15, 19

And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering.

And I have given the Levites as a gift to Aaron and his sons from among the people of Israel, to do the service for the people of Israel at the tent of meeting and to make atonement for the people of Israel, that there may be no plague among the people of Israel when the people of Israel come near the sanctuary.”