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: 5963
Abdullah b. Ja'far reported that one day Allah's Messenger (may peace be upon him) mounted me behind him and narrated to me something in secret which I would narrate to none amongst people.