Sahih al-Bukhari 3718
صحيح البخاري#3718SahihNarrated 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 62:66