Joshua 8:12 (KJV)

And Joshua rose up early in the morning, and numbered the people, and went up, he and the elders of Israel, before the people to Ai.

And all the people, even the people of war that were with him, went up, and drew nigh, and came before the city, and pitched on the north side of Ai: now there was a valley between them and Ai.

And he took about five thousand men, and set them to lie in ambush between Beth–el and Ai, on the west side of the city.

And when they had set the people, even all the host that was on the north of the city, and their liers in wait on the west of the city, Joshua went that night into the midst of the valley.

And it came to pass, when the king of Ai saw it, that they hasted and rose up early, and the men of the city went out against Israel to battle, he and all his people, at a time appointed, before the plain; but he wist not that there were liers in ambush against him behind the city.