Numbers 8:19, 22

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.”

And after that the Levites went in to do their service in the tent of meeting before Aaron and his sons; as the LORD had commanded Moses concerning the Levites, so they did to them.