Sahih Bukhari Volume 005, Book 057, Hadith Number 063.
Narated By Marwan bin Al-Hakam : 'Uthman bin 'Affan was afflicted with severe nose-bleeding in the year when such illness was prevelant and that prevented him from performing Hajj, and (because of it) he made his will. A man from Quraish came to him and said, "Appoint your successor." 'Uthman asked, "Did the people name him? (i.e. the successor) the man said, "Yes." Uthman asked, "Who is that?" The man remained silent. Another man came to 'Uthman and I think it was Al-Harith. He also said, "Appoint your successor." 'Uthman asked, "Did the people name him?" The man replied "Yes." 'Uthman said, "Who is that?" The man remained silent. 'Uthman said, "Perhaps they have mentioned Az-Zubair?" The man said, "Yes." 'Uthman said, "By Him in Whose Hands my life is, he is the best of them as I know, and the dearest of them to Allah's Apostle."