Sahih Bukhari
Sahih Bukhari Volume 009, Book 089, Hadith Number 254.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 89. Judgments (Ahkaam)
Narrated By Ibn 'Umar : Allah's Apostle said, "This matter (caliphate) will remain with the Quraish even if only two of them were still existing."