Sahih al-Bukhari Book 78, Hadith 61
The Prophet (ﷺ) was not one who would abuse (others) or say obscene words, or curse (others), and if he
wanted to admonish anyone of us, he used to say: "What is wrong with him, his forehead be dusted!"
`Aisha said that the Jews came to the Prophet (ﷺ) and said, "As-Samu 'Alaikum" (death be on you). `Aisha said (to them), "(Death) be on you, and may Allah curse you and shower His wrath upon you!" The Prophet (ﷺ) said, "Be calm, O `Aisha ! You should be kind and lenient, and beware of harshness and Fuhsh (i.e. bad words)." She said (to the Prophet), "Haven't you heard what they (Jews) have said?" He said, "Haven't you heard what I have said (to them)? I said the same to them, and my invocation against them will be accepted while theirs against me will be rejected (by Allah). "
A man asked permission to enter upon the Prophet. When the Prophet (ﷺ) saw him, he said, "What an evil brother of his tribe! And what an evil son of his tribe!" When that man sat down, the Prophet (ﷺ) behaved with him in a nice and polite manner and was completely at ease with him. When that person had left, 'Aisha said (to the Prophet). "O Allah's Apostle! When you saw that man, you said so-and-so about him, then you showed him a kind and polite behavior, and you enjoyed his company?" Allah's Messenger (ﷺ) said, "O 'Aisha! Have you ever seen me speaking a bad and dirty language? (Remember that) the worst people in Allah's sight on the Day of Resurrection will be those whom the people leave (undisturbed) to be away from their evil (deeds)."