Sahih Bukhari Volume 8, Book 73, Hadith Number 152.

Narated By ‘Aisha : A man asked permission to see the Prophet. He said, “Let Him come in; What an evil man of the tribe he is! (Or, What an evil brother of the tribe he is).” But when he entered, the Prophet spoke to him gently in a polite manner. I said to him, “O Allah’s Apostle! You have said what you have said, then you spoke to him in a very gentle and polite manner? The Prophet said, “The worse people, in the sight of Allah are those whom the people leave (undisturbed) to save themselves from their dirty language.”