Numbers 1:51-53

⁵¹And in removing the tabernacle, the Levites shall take it down, and in pitching the tabernacle they shall set it up: and let the stranger that advances to touch it die.

⁵²And the children of Israel shall encamp, every man in his own order, and every man according to his company, with their host.

⁵³But let the Levites encamp round about the tabernacle of witness fronting it, and so there shall be no sin amongst the children of Israel; and the Levites themselves shall keep the guard of the tabernacle of witness.