Numbers 1:47-51

But the Levites after the tribe of their fathers were not numbered among them. For the LORD had spoken unto Moses, saying, Only thou shalt not number the tribe of Levi, neither take the sum of them among the children of Israel: but thou shalt appoint the Levites over the tabernacle of testimony, and over all the vessels thereof, and over all things that belong to it: they shall bear the tabernacle, and all the vessels thereof; and they shall minister unto it, and shall encamp round about the tabernacle. And when the tabernacle setteth forward, the Levites shall take it down: and when the tabernacle is to be pitched, the Levites shall set it up: and the stranger that cometh nigh shall be put to death.