Yahya related to me from Malik from Abu'z Zinad from al-A'rajfrom Abu Hurayra that the Messenger of Allah, may Allah bless him andgrant him peace, said, "Fasting is a protection for you, so when youare fasting, do not behave obscenely or foolishly, and if any oneargues with you or abuses you, say, 'I am fasting. I am fasting.' "
USC-MSA web (English) reference: Book 18, Hadith 57
Arabic reference: Book 18, Hadith 690