Rashi on Numbers 23:7/רש"י על במדבר כ״ג:ז׳

1ארה לי יעקב ולכה זעמה ישראל CURSE JACOB FOR ME AND COME EXECRATE ISRAEL — by their two names did he bid him curse them, for perhaps one of them was not distinctive enough to point them out beyond all doubt as being the people against whom the curse was directed.