Sahih Bukhari Volume 009, Book 089, Hadith Number 324.
Narrated By Al-Qasim bin Muhammad : 'Aisha said, "O my head!" Allah's Apostle said, "If that (i.e., your death) should happen while I am still alive, I would ask Allah to forgive you and would invoke Allah for you." 'Aisha said, "O my life which is going to be lost! By Allah, I think that you wish for my death, and if that should happen then you would be busy enjoying the company of one of your wives in the last part of that day." The Prophet said, "But I should say, 'O my head!' I feel like calling Abu Bakr and his son and appoint (the former as my successors lest people should say something or wish for something. Allah will insist (on Abu Bakr becoming a Caliph) and the believers will prevent (anyone else from claiming the Caliphate)," or "...Allah will prevent (anyone else from claiming the Caliphate) and the believers will insist (on Abu Bakr becoming the Caliph)."