Rashi on Numbers 23:16/רש"י על במדבר כ״ג:ט״ז

1וישם דבר בפיו AND HE PUT A WORD IN HIS MOUTH —And what was this “putting”? What would Scripture have lacked if it had stated “[And the Lord met Balaam and said,] Return to Balak, and thus shalt thou speak”? But the explanation is: When he heard that he was not permitted to curse, he said, “Why should I return to Balak to cause him annoyance?” Therefore the Holy One, blessed be He, put a halter and hook in his mouth as a person who pricks an animal with a hook to make it go wherever he wants. He said to him, “Despite yourself shall you return to Balak’ (cf. Sanhedrin 105b; Midrash Tanchuma, Balak 13).