Joshua 4:6-7

"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" - Joshua 4:6-7 KJV