Numbers 18:23

But the Levites shall do the [menial] service of the Tent of Meeting, and they shall bear and remove the iniquity of the people [that is, be answerable for the legal pollutions of the holy things and offer the necessary atonements for unintentional offenses in these matters]. It shall be a statute forever in all your generations, that among the Israelites the Levites have no inheritance [of land].