1 Samuel 14:21-22

21Now the Hebrews who had been with the Philistines before that time and who had gone up with them into the camp, even they also turned to be with the Israelites who were with Saul and Jonathan.

22Likewise, when all the men of Israel who had hidden themselves in the hill country of Ephraim heard that the Philistines were fleeing, they too followed hard after them in the battle.