Numbers 18:1-7

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. They shall join you and attend to the duties of the Tent of Meeting—all the service of the tent—and no stranger [no layman, anyone who is not a Levite] may approach you [and your sons]. So you shall attend to the duties of the sanctuary and the duties of the altar [of burnt offering and the altar of incense], so that there will no longer be wrath on the Israelites [as with Korah, Dathan, and Abiram]. [Num 16:42-50] Behold, I Myself have taken your fellow Levites from among the sons of Israel; they are a gift to you, given (dedicated) to the LORD, to do the service for the Tent of Meeting (tabernacle). Therefore you and your sons with you shall attend to your priesthood for everything concerning the altar [of burnt offering and the altar of incense] and [of the Holy of Holies] within the veil, and you are to serve. I am giving you the priesthood as a bestowed service, but the stranger (outsider, layman) who approaches shall be put to death.” [Ex 40:18, 20, 26]