Joshua 4:4-6

Then Joshua called the twelve men, whom he had prepared of the children of Israel, out of every tribe a man: and Joshua said unto them, Pass over before the ark of the LORD your God into the midst of Jordan, and take you up every man of you a stone upon his shoulder, according unto the number of the tribes of the children of Israel: that this may be a sign among you, that when your children ask their fathers in time to come, saying, What mean ye by these stones?