The Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not give up false speech and acting upon it, Allaah has no need of his giving up his food and drink.” Narrated by al-Bukhaari, 1903, 6075.
False speech includes all kinds of speech that are haraam, such as lying, backbiting, slandering, insulting and cursing.
And the Prophet (peace and blessings of Allaah be upon him) said: “If any one of you is fasting, let him not utter obscenities or act in an ignorant manner, and if anyone insults him or wants to fight him, let him say, I am fasting.” Narrated by al-Bukhaari, 1894; Muslim 1151.