Numbers 8:19, 22

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.”

After that the Levites went in to do their work in the tabernacle of meeting before Aaron and his sons; as the LORD commanded Moses concerning the Levites, so they did to them.