Joshua 4:8 (KJV)

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.

And Joshua set up twelve stones in the midst of Jordan, in the place where the feet of the priests which bare the ark of the covenant stood: and they are there unto this day.

For the priests which bare the ark stood in the midst of Jordan, until every thing was finished that the Lord commanded Joshua to speak unto the people, according to all that Moses commanded Joshua: and the people hasted and passed over.