Sahih Bukhari Volume 005, Book 057, Hadith Number 064.

Narated By Marwan bin Al-Hakam : While I was with 'Uthman, a man came to him and said, "Appoint your successor." 'Uthman said, "Has such successor been named?" He replied, "Yes, Az-Zubair." 'Uthman said, thrice, "By Allah! Indeed you know that he is the best of you."