Numbers 18:2-4

2But 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].

3And 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.

4They 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].