Numbers 8:6-11, 21

“Take the Levites away from the other Israelites and make them clean. This is what you should do to make them clean: Sprinkle the cleansing water on them, and have them shave their bodies and wash their clothes so they will be clean. They must take a young bull and the grain offering of flour mixed with oil that goes with it. Then take a second young bull for a sin offering. Bring the Levites to the front of the Meeting Tent, and gather all the Israelites around. When you bring the Levites before the LORD, the Israelites should put their hands on them. Aaron will present the Levites before the LORD as an offering presented from the Israelites. Then the Levites will be ready to do the work of the LORD.

The Levites made themselves clean and washed their clothes. Then Aaron presented them as an offering to the LORD. He also removed their sins so they would be pure.