From Sahih Muslim (Hadith 6033):
Masruq reported:We went to Abdullah b. 'Amr when Mu'dwiya came to Kufa, and he made a mention of Allah's Messenger (ﷺ) and said: He was never immoderate in his talk and he never reviled others. Allah's Messenger (ﷺ) 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)
Reference: Muslim 6033