From Sahih Muslim (Hadith 6216):
Sa'id b. al-Musayyib reported Abu Musa Ash'ari having said that Allah's Messenger (ﷺ) 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)
Reference: Muslim 6216