Numbers 8:20-22

So Moses, Aaron, and the whole community of Israel dedicated the Levites, carefully following all the LORD’s instructions to Moses. The Levites purified themselves from sin and washed their clothes, and Aaron lifted them up and presented them to the LORD as a special offering. He then offered a sacrifice to purify them and make them right with the LORD. 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.