Rashi on Numbers 22:9/רש"י על במדבר כ״ב:ט׳

1מי האנשים האלה עמך WHO ARE THESE MEN WITH THEE? — By putting this question He intended to delude him. And indeed he (Balaam) thereupon said: “It seems, then, that there are times when everything is not manifest to Him; His knowledge is not always alike. I will select a time when I can curse and when He will not observe it” (Midrash Tanchuma, Balak 5).