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.

Joshua 4:21-22

4:21 And he spake unto the children of Israel, saying, When your children shall ask their fathers in time to come, saying, What [mean] these stones? 4:22 Then ye shall let your children know, saying, Israel came over this Jordan on dry land.