And God said to Bilam: You shall not go with them; you shall not curse the people, for it is blessed.
Bamidbar Rabbah 20:9–10 — the double prohibition and the blessed status of Israel.
English: “You shall not go with them — you shall not curse the people, for it is blessed.” He said to him: If you seek to curse, I will not allow you; for they are blessed from the Patriarchs.
Aggadic expansion of the divine refusal.
The refusal is twofold — neither accompany the mission nor utter the curse — because Israel’s blessing is already sealed from the Patriarchs.