Joshua 4:4-7

So Joshua chose one man from each tribe. Then he called the twelve men together and said to them, “Go out into the river where the Ark of the LORD your God is. Each of you bring back one rock, one for each tribe of Israel, and carry it on your shoulder. They will be a sign among you. In the future your children will ask you, ‘What do these rocks mean?’ Tell them the water stopped flowing in the Jordan when the Ark of the Agreement with the LORD crossed the river. These rocks will always remind the Israelites of this.”