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: 5912
Sa'id b. al-Musayyib reported Abu Musa Ash'ari having said that Allah's Messenger (may peace be upon him) set out one day to the suburbs of Medina for reliev- ing himself. I followed his steps. The rest of the hadith is the same. Ibn Musayyib said: I concluded (from the manner of their sitting) the (order) of their graves. (The three) would be together (the graves of the Holy Prophet, Hadrat Abu Bakr and Hadrat Umar) and that of 'Uthman would be separate (from them).