Numbers 23:5

And the Lord put a word in Bileam's mouth, and said, Return to Balak, and thus speak.

AND THE ETERNAL PUT A WORD IN BALAAM’S MOUTH. Some commentators explain that Balaam did not understand the words [he said], but G-d filled him with words and said to him: “Return unto Balak, and thus thou shalt speak, for the words will come forth [automatically] from your mouth.” Perhaps this is also the opinion of our Rabbis, who have said: 12; Bamidbar Rabbah 20:16. “He shaped his mouth and formed it [so that he would speak as He desired], like a person who fixes a nail onto a board. Rabbi Eleazar says: It was an angel that spoke: Rabbi Yehoshua says: etc.”). But it does not appear to me to be correct [that Balaam did not know what he was saying], because he said [that he] heareth the words of G-d; seeth the vision of the Almighty. But the meaning of [the word] ‘vayasem’ (and He put) is “instruction,” signifying He taught him the words so that he should recite them with his mouth, and he should not forget or omit any part of it, similar to [the expression], teach thou it the children of Israel; ‘simah’ (put it) in their mouths. Similarly, for by the appointment of Absalom this hath been ‘sumah’ (‘put; determined’).

כה תדבר, “G–d commenced with the same word with which He had announced to Avraham that he would become the founding father of a nation, though at that time he had not yet been able to father a single child. [Compare Genesis 15,5, a conversation between G–d and Avraham which had preceded the command for him to leave Charan in chapter 12. Ed.] He reminded Bileam by referring to that promise that any attempt by him to curse Avraham’s descendants would be futile. In fact G–d had guaranteed to Avraham that his descendants would increase and multiply.