"Come, curse Jacob for me" (Numbers 23:7). Had you told me to curse another nation, such as [the children of] Abraham and Isaac, I would have been able to do so. But Jacob? When a king selects a portion for himself, and another comes and speaks disgrace against it, does he keep his life? Now these are His inheritance, His portion, and His treasure, as it is said, "For the LORD's portion is His people, Jacob the lot of His inheritance" (Deuteronomy 32:9), "and you shall be My treasure from among all the peoples" (Exodus 19:5).
Why Balaam Could Not Curse Jacob, the King's Own Portion
Midrash Tanchuma Buber, Balak 17:6
לכה ארה לי יעקב. אילו אומה אחרת אמרת לי לקלל, כגון [בני] אברהם ויצחק הייתי יכול, אלא יעקב, מלך שבורר לעצמו מנה ועמד אחר ואמר גנאי עליה, כלום יש לו חיים, ואלו נחלתו וחבלתו וסגולתו, שנאמר כי חלק ה' עמו יעקב חבל נחלתו (דברים לב ט), והייתם לי סגולה מכל העמים וגו' (שמות יט ה).