Numbers 4:5-6

5When the camp is to move, Aaron and his sons are to go in and take down the shielding curtain and put it over the ark of the covenant law.

6Then they are to cover the curtain with a durable leather, Possibly the hides of large aquatic mammals; also in verses 8, 10, 11, 12, 14 and 25 spread a cloth of solid blue over that and put the poles in place.