Musnad Ahmad 32:1177

Mu'adh ibn Mu'adh Abu al-Muthanna narrated to us, Sulayman al-Taymi narrated to us, from Abu Mijlaz, from Anas ibn Malik, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed Qunut for a month after the bowing (ruku'), supplicating against [the tribes of] Ri'l and Dhakwan."