Numbers 8:19, 22

And of all the Israelites, I have assigned the Levites to Aaron and his sons. They will serve in the Tabernacle on behalf of the Israelites and make sacrifices to purify the people so no plague will strike them when they approach the sanctuary.”

After that the Levites went into the Tabernacle to perform their duties, assisting Aaron and his sons. So they carried out all the commands that the LORD gave Moses concerning the Levites.