Numbers 8:15, 19

After that the Levites shall go in to service the tabernacle of meeting. So you shall cleanse them and offer them like a wave offering.

And I have given the Levites as a gift to Aaron and his sons from among the children of Israel, to do the work for the children of Israel in the tabernacle of meeting, and to make atonement for the children of Israel, that there be no plague among the children of Israel when the children of Israel come near the sanctuary.”