Sahih Bukhari

Narrated 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