Numbers 8:13-15

Then have the Levites stand in front of Aaron and his sons, and raise your hands and present them as a special offering to the LORD. In this way, you will set the Levites apart from the rest of the people of Israel, and the Levites will belong to me. After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering.