Sayyiduna Kurayz bin Samah (may Allah be pleased with him), who had come to the Prophet (peace and blessings be upon him) in a delegation, said: It was said: O Messenger of Allah, invoke a curse upon Banu Amir. He (peace and blessings be upon him) said: "I have not been sent as one who curses."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16473