Numbers 8:15, 19

Then after that the Levites may go in to serve the tent of meeting. But you shall cleanse them and present them as a wave offering

I have given the Levites as a gift to Aaron and to his sons from among the sons of Israel, to perform the service of the sons of Israel at the tent of meeting and to make atonement on behalf of the sons of Israel, so that there will be no plague among the sons of Israel by their coming near to the sanctuary.”