Numbers 8:20 KJV

18And I have taken the Levites for all the firstborn of the children of Israel.

19And I have given the Levites as a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the tabernacle of the congregation, and to make an atonement for the children of Israel: that there be no plague among the children of Israel, when the children of Israel come nigh unto the sanctuary.

20And Moses, and Aaron, and all the congregation of the children of Israel, did to the Levites according unto all that the LORD commanded Moses concerning the Levites, so did the children of Israel unto them.

21And the Levites were purified, and they washed their clothes; and Aaron offered them as an offering before the LORD; and Aaron made an atonement for them to cleanse them.

22And after that went the Levites in to do their service in the tabernacle of the congregation before Aaron, and before his sons: as the LORD had commanded Moses concerning the Levites, so did they unto them.