Musnad Abu Ya'la al-Mawsili 110:193

Abu Bakr ibn Abi Shaybah narrated to us, Hamid ibn Abd al-Rahman narrated to us, from Abdullah ibn al-Mu'ammal, from Ata, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the people differ, the truth is with Mudar. And when Rabi'ah becomes mighty, that is the humiliation of Islam."