Chapter 45: The Book of the Merits of the Companions
Ibn Abu Mulaika reported: I heard 'A' isha as saying and she was asked as to whom Allah' s Messenger (ﷺ) would have nominated as 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. al-Jarrah, and then she kept quiet at this.
Sahih Muslim 2385