Musnad Ahmad 2:218

Muhammad ibn Bishr narrated to us, Hisham ibn Urwah narrated to us, from his father, from Ibn Umar: That it was said to Umar, "Will you not appoint a successor?" He said, "If I leave it, then one who is better than me left it: the Messenger of Allah, may Allah bless him and grant him peace. And if I appoint a successor, then one who is better than me appointed a successor: Abu Bakr."