Hadith 1240

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) 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].