Joshua 4:4-6

Then Joshua called the twelve men whom he had appointed from the children of Israel, one man from every tribe; 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?’