Joshua 4:5-7

and Joshua said to them: “Cross over before the ark of the LORD your God into the midst of the Jordan, and each one of you take up a stone on his shoulder, according to the number of the tribes of the children of Israel, that this may be a sign among you when your children ask in time to come, saying, ‘What do these stones mean to you?’ Then you shall answer them that the waters of the Jordan were cut off before the ark of the covenant of the LORD; when it crossed over the Jordan, the waters of the Jordan were cut off. And these stones shall be for a memorial to the children of Israel forever.”