Musnad Abu Ya'la al-Mawsili 153:188

Abu Ma'mar Isma'il ibn Ibrahim narrated to us, Ibn al-Majishun Yusuf narrated to us, from his father, from Ibn 'Umar, who said: "The Messenger of Allah, may Allah bless him and grant him peace, was [the best], and we would not equate anyone with him. Then we would say: the best of the people is Abu Bakr, then 'Umar, then 'Uthman, and then we would not rank [anyone else] above another."