Numbers 18:2-4

Amplified Bible

² 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]. ³ 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. ⁴ 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].