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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."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنِي عُبَيْدُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، أَخْبَرَنِي أَبِي، سَمِعْتُ مَرْوَانَ، كُنْتُ عِنْدَ عُثْمَانَ، أَتَاهُ رَجُلٌ فَقَالَ اسْتَخْلِفْ. قَالَ وَقِيلَ ذَاكَ قَالَ نَعَمْ، الزُّبَيْرُ. قَالَ أَمَا وَاللَّهِ إِنَّكُمْ لَتَعْلَمُونَ أَنَّهُ خَيْرُكُمْ. ثَلاَثًا.
sunnah.com