Joshua 4:6-7

4:6 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? 4:7 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.