Numbers 8:19

And 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, that which should have been performed by them in their first-born, 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. By their representative service the Levites constituted an atonement for the children of Israel, for the latter were restrained by this arrangement from approaching near to the Sanctuary and thus bringing a calamity upon themselves.