Numbers 1:51-53

⁵¹When you are to go forward, the Levites shall take down the tabernacle: when you are to camp, they shall set it up. What stranger soever cometh to it, shall be slain.

⁵²And the children of Israel shall camp every man by his troops and bands and army.

⁵³But the Levites shall pitch their tents round about the tabernacle, lest there come indignation upon the multitude of the children of Israel, and they shall keep watch, and guard the tabernacle of the testimony.