And the Prophet Muhammad (peace and blessings be upon him) said: Fasting is a shield. When any one of you wakes up in the morning in a state of fasting, he should not utter any foolish or ignorant words. Rather, if someone fights with him or abuses him, he should simply say: I am fasting.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8128