Joshua 4:5-7

5And Joshua said to them, Pass over before the ark of the LORD your God into the midst of Jordan, and take ye up every man of you a stone upon his shoulder, according to the number of the tribes of the children of Israel:

6That 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?1

7Then 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 to the children of Israel for ever.