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: 5873
'Abdullah reported Allah's Messenger (may peace be upon him) as saying: If I were to choose as my bosom friend I would have chosen the son of Abu Quhafa (Abu Bakr) as my bosom friend.