Numbers 4:4-6

This {shall be} the service of the sons of Kohath in the tabernacle of the congregation, {about} the most holy things: 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: And shall put thereon the covering of badgers' skins, and shall spread over {it} a cloth wholly of blue, and shall put in the staves thereof.