Numbers 18:1-3

So the LORD said to Aaron, “You and your sons and your father’s household (family) with you shall bear the guilt in connection with the sanctuary [that is, through your service as priests you will atone for the offenses which the people unknowingly commit when brought into contact with the manifestations of God’s presence]; and you and your sons with you shall bear the guilt in connection with your priesthood [that is, your own unintentional offenses]. But bring with you also your brothers, the tribe of Levi, the tribe of your father (ancestor), so that they may join with you and serve you [as assistants], while you and your sons with you are before the Tent of the Testimony [the Holy Place where only priests may go, and the Most Holy Place which only the high priest may enter]. And the Levites shall attend to your duty [as assistants] and to the duties of all the tent (tabernacle); only they shall not approach the articles of the sanctuary and the altar, or both they and you will die.