Joshua 4:5-7

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? Then ye shall answer them, That the waters of Jordan were cut off before the ark of the covenant of the LORD; when it passed over Jordan, the waters of Jordan were cut off: and these stones shall be for a memorial unto the children of Israel for ever.