Numbers 4:4-5

4:4 This [shall be] the service of the sons of Kohath in the tabernacle of the congregation, [about] the most holy things: 4:5 And when the camp setteth forward, Aaron shall come, and his sons, and they shall take down the covering vail, and cover the ark of testimony with it:

Numbers 4:15

4:15 And when Aaron and his sons have made an end of covering the sanctuary, and all the vessels of the sanctuary, as the camp is to set forward; after that, the sons of Kohath shall come to bear [it]: but they shall not touch [any] holy thing, lest they die. These [things are] the burden of the sons of Kohath in the tabernacle of the congregation.

Numbers 4:20

4:20 But they shall not go in to see when the holy things are covered, lest they die.