Rashi on Numbers 22:4/רש"י על במדבר כ״ב:ד׳

1אל זקני מדין AND MOAB SAID] TO THE ELDERS OF MIDIAN But did not these (Moab and Midian) always hate one another, just as is stated, (Genesis 36:35) “who had smitten Midian in the country of Moab”, from which it is evident that Midian had come against Moab in war? But out of fear of Israel they now made peace between themselves (Midrash Tanchuma, Balak 3; cf. Sanhedrin 105a). And what induced Moab to take counsel of Midian? When they saw that Israel was victorious in a supernatural manner they said: the leader of these people grew up in Midian; let us ask them what is his chief characteristic. They replied to them; “His power lies only in his mouth (in prayer)”. Whereupon they said: “Then we must come against them with a man whose power lies in his mouth” (Midrash Tanchuma, Balak 3).

2כלחך השור AS THE OX LICKETH — they used this comparison because whatever piece of ground the ox licks has no blessing left in it (cf. Shabbat 140b, 141a; Midrash Tanchuma, Balak 3).

3בעת ההוא [BALAK WAS KING OF MOAB] AT THAT TIME — at that time: he was not really entitled to the kingdom; he was actually one of the Midianite princes, but when Sihon died they (the Moabites) appointed him as king over them to meet the needs of the time (Midrash Tanchuma, Balak 4).