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: 5872
Abdullah reported Allah's Messenger (may peace be upon him) as saying: If I were to choose from my Umma anyone as my bosom friend, I would have chosen Abu Bakr.