Imam Al-Nawawi's Riyad-us-Saliheen
Chapter 223
Safeguarding As-Saum (The Fast)

1240. Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "When any one of you is observing Saum (fasting) on a day, he should neither indulge in obscene language nor should he raise the voice; and if anyone reviles him or tries to quarrel with him he should say: `I am observing fast.'''
[Al-Bukhari and Muslim].

Commentary:  This Hadith has already been mentioned and is repeated here for its being relevant to this chapter. We learn from it that during Saum not only food, drink and sex are prohibited but the improper use of the tongue and the other organs of the body as well. When one is observing Saum, he should neither use abusive language nor talk foolishly nor tell lies nor make obscene conversation nor indulge in backbiting nor quarrel with anyone. If someone provokes him, he should keep himself quiet and remember that he is observing Saum and he has to abstain from such things. As far as possible, he should keep his tongue engaged in the remembrance of Allah and recitation of the Qur'an.

1241. Abu Hurairah (May Allah be pleased with him) reported: The Prophet (sallallaahu ’alayhi wa sallam) said, "If one does not eschew lies and false conduct, Allah has no need that he should abstain from his food and his drink.''
[Al-Bukhari].

Commentary: This Hadith exhorts those observing Saum to fulfill all the requirements of fasting. One should not conduct himself in such a manner that on the one side he observes Saum and on the other he is fearless of Allah. To save himself from Allah's Wrath and to get the reward of the fasts, one must abstain from all sorts of vices, such as cheating, lying, backbiting, and using obscene language. The threat held against such people in this Hadith should make them fear that their Saum will go waste and they would be deprived of its reward. It does not mean that such people should start eating and drinking during Saum but what is intended is that they should save themselves from all kinds of sins so that they may earn the reward of Saum.