Joshua 8:2-4

2And thou shalt do to Ai and to its king as thou didst to Jericho and to its king; only, the spoil thereof and the cattle thereof shall ye take as prey for yourselves. Set an ambush against the city behind it.

3And Joshua arose, and all the people of war, to go up to Ai. And Joshua chose thirty thousand valiant men, and sent them away by night.

4And he commanded them, saying, See, ye shall be in ambush against the city, behind the city: go not very far from the city, and be all of you ready.