Musnad Ahmad 26:289

Muhammad ibn Sabiq narrated to us, Isra'il narrated to us, from al-A'mash, from Ibrahim, from 'Alqamah, from 'Abdullah ibn Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The believer is not one who attacks (others' character), nor is he one who curses, nor is he obscene or foul-mouthed." And Ibn Sabiq said once: "The attacker, nor the curser."