When the princes of Moav asked Bilam to curse Israel, he informed them that he would wait for instructions from God. Indeed, God instructed him not to go. When they insisted he come, God agreed he could accompany them, provided that he would say only the prophecies that God would give him. An angel of God impeded the progress of his donkey. Bilam, unaware of the angel, tried to force the donkey to continue, and the donkey spoke with Bilam and rebuked him. The angel, too, repeated the message that Bilam would be compelled to repeat with precision the prophecies he would receive from God. The Sages describe this episode.

“God said to Bilam: You shall not go with them” (Numbers 22:12). Bilam said to Him: If so, I will curse them from my place. God said to him: “You shall not curse the people” (22:12). Bilam said to Him: If so, I will bless them. God said to him: They do not need your blessing, “as it is blessed” (22:12). As people say to the wasp: We want not from your honey and not from your sting…. [The verses recount the conversation between Bilam and the donkey:] “It said to Bilam: What did I do to you that you struck me these three times?” (22:28). [By mentioning three, the donkey] alluded to him: You are seeking to uproot a nation that celebrates three festivals. Bilam responded: “If only there were a sword in my hand, I would have killed you now” (22:29). This can be explained by means of a parable of a doctor who came to cure someone with a snakebite using the speech of his tongue. On the way, he encountered a certain large lizard. He began looking for a stick to kill it. People said: You are unable to kill this lizard without a stick; how do you come to cure one with a snakebite using your tongue? Likewise, the donkey said to Bilam: You are unable to kill me without a sword in your hand; how do you hope to uproot an entire nation? Bilam was silent and did not find an answer. The princes of Moav began doubting him, as they saw an unprecedented miracle [that the donkey spoke]. (Bemidbar Rabba 20)