Numbers 4:5

and when the camp setteth. forward, the signal having been given to move on to the next station, Aaron shall come, and his sons, for only the priests were allowed to perform this particular service, and they shall take down the covering veil, the screen of the Most Holy Place, and cover the Ark of the Testimony with it, this curtain thus serving as a screen even during the journey, for the very looking at the ark in presumptuous curiosity was punished by the Lord, 1 Samuel 6:19;