Numbers 8:19
And I have given the Levites as a gift to Aaron and his sons from among the Israelites, to perform the service for the Israelites at the Tent of Meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary."

Numbers 8:21
The Levites purified themselves and washed their clothes, and Aaron presented them as a wave offering before the LORD. Aaron also made atonement for them to cleanse them.