Numbers 8:19, 22

From among all the Israelites, I have given the Levites as gifts to Aaron and his sons to do the work at the tent of meeting on behalf of the Israelites and to make atonement for them so that no plague will strike the Israelites when they go near the sanctuary.”

After that, the Levites came to do their work at the tent of meeting under the supervision of Aaron and his sons. They did with the Levites just as the LORD commanded Moses.