Rashi on Genesis 36:35/רש"י על בראשית ל״ו:ל״ה

1המכה את מדין בשדה מואב WHO SMOTE MIDIAN IN THE FIELD OF MOAB — for Midian came to war against Moab and the king of Edom went to assist Moab. From here we learn that although Midian and Moab were at strife one with the other yet in the time of Balaam they made peace in order to band themselves against Israel (Midrash Tanchuma, Balak 3).