Numbers 8:19, 22

And 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 affliction among the sons of Israel due to their approaching the sanctuary.”

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