Joshua 4:6 (KJV)

Then Joshua called the twelve men, whom he had prepared of the children of Israel, out of every tribe a man:

And Joshua said unto 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 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.

And the children of Israel did so as Joshua commanded, and took up twelve stones out of the midst of Jordan, as the Lord spake unto Joshua, according to the number of the tribes of the children of Israel, and carried them over with them unto the place where they lodged, and laid them down there.