Jdt 11:17-18

17Your servant is indeed God-fearing and serves the God of heaven night and day. So, my lord, I will remain with you; but every night your servant will go out into the valley and pray to God. He will tell me when they have committed their sins.18Then I will come and tell you, so that you may go out with your whole army, and not one of them will be able to withstand you.