Imam Al-Nawawi’s Riyad-us-Saliheen
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].
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.”