Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5877
Ibn Abu Mulaika reported: I heard A'isha 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.