Musnad Abu Ya'la al-Mawsili 135:9

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, 'Abd al-Wahid narrated to us, 'Asim al-Ahwal narrated to us, he said: I asked Anas ibn Malik about the Qunut. He said: The Messenger of Allah (may Allah bless him and grant him peace) performed Qunut for a month after the bowing (ruku'). He had sent seventy men one day to a people among the polytheists. He said: And there was a people among the polytheists who had a covenant between them and the Messenger of Allah (may Allah bless him and grant him peace). The polytheists with whom there was a covenant between them and the Messenger of Allah (may Allah bless him and grant him peace) killed them. So the Messenger of Allah (may Allah bless him and grant him peace) performed Qunut for a month, supplicating against them.