Musnad Abu Ya'la al-Mawsili 152:311

Abu Khaythama narrated to us, Ahmad ibn Yunus narrated to us, Abu Bakr ibn Ayyash narrated to us, from al-Hasan ibn Amr al-Fuqami, from Muhammad ibn Abd al-Rahman ibn Yazid, from his father, from Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The believer is not one who attacks (others' character), nor one who curses, nor one who is obscene, nor one who is foul-mouthed."