Musnad Ahmad 1153:337

Waki' narrated to us, Abu al-Umays narrated to us, from Ibn Abi Mulaykah, from Aisha, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) passed away and did not appoint a successor. And had he been one to appoint a successor, he would have appointed Abu Bakr or Umar."