Joshua 8:30

And the LORD said unto Joshua, Fear not, neither be thou dismayed: take all the people of war with thee, and arise, go up to Ai: see, I have given into thy hand the king of Ai, and his people, and his city, and his land:

And thou shalt do to Ai and her king as thou didst unto Jericho and her king: only the spoil thereof, and the cattle thereof, shall ye take for a prey unto yourselves: lay thee an ambush for the city behind it.

So Joshua arose, and all the people of war, to go up against Ai: and Joshua chose out thirty thousand mighty men of valour, and sent them away by night.

And he commanded them, saying, Behold, ye shall lie in wait against the city, even behind the city: go not very far from the city, but be ye all ready: