Numbers 18:3-5

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]