Joshua 3:3-4

and they commanded the people, “When you see the ark of the covenant of the LORD your God being carried by the Levitical priests, then you shall set out from where you are and follow it. However, there shall be a distance between you and it of about 2,000 cubits (3,000 ft.) by measurement. Do not come near it, so that you may [be able to see the ark and] know the way you are to go, for you have not passed this way before.”