Sahih Muslim Book 31, Hadith Number 5877.
Chapter : The merits of Abu Bakr Siddiq (Allah be pleased with him).
Ibn Abu Mulaika reported: I heard ‘Aisha as saying and she was asked as to whom Allah’s Messenger (may peace be upon him) would have nominated his successor if he had to nominate one at all. She said: Abu Bakr. It was said to her: Then whom after Abu Bakr? She said: Umar. It was said to her. Then whom after ‘Umar? She said: Abu Ubaida b. Jarrab, and then she kept quiet at this.