Musnad Abu Ya'la al-Mawsili 115:228

Ubayd Allah narrated to us, Yazid bin Zuray' narrated to us, Hisham al-Dastawa'i narrated to us, Qatadah narrated to us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) performed Qunut for a month after the bowing (ruku'), supplicating against tribes from the tribes of the Arabs.