Numbers 8:15, 19

After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering.

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.”