Musnad Ahmad 2:250

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Salim, from Ibn 'Umar, that he said to 'Umar: "I heard the people saying a statement, so I swore that I would say it to you. They claim that you are not appointing a successor." He lowered his head for a while, then he raised it and said: "Indeed, Allah the Almighty and Majestic preserves His religion. If I do not appoint a successor, then the Messenger of Allah (peace and blessings of Allah be upon him) did not appoint a successor. And if I do appoint a successor, then Abu Bakr did appoint a successor." He (Ibn 'Umar) said: "By Allah, it was nothing but that he mentioned the Messenger of Allah (peace and blessings of Allah be upon him) and Abu Bakr, so I knew that he would not equate anyone with the Messenger of Allah (peace and blessings of Allah be upon him), and that he would not appoint a successor."