Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5740
Masruq reported: We went to Abdullah b. 'Amr when Mu'dwiya came to Kufa, and he made a mention of Allah's Messenger (may peace be upon him) and said: He was never immoderate in his talk and he never reviled others. Allah's Messenger (may peace be upon him) also said: The best amongst you are those who are best in morals. Uthman said: When he came to Kufa along with Mu'awiya... (The rest of the hadith is the same).