Charles Ellicott Commentary Numbers 1:51

"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." — Numbers 1:51 (ASV)

And the stranger that comes near shall be put to death. —The word zar (stranger) here denotes one who was not of the tribe of Levi (Leviticus 22:10; Leviticus 22:12).