It is narrated from Hazrat Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: When any one of you wakes up in the morning in a state of fasting, he should not utter any foolish or ignorant words, and if someone wants to fight or abuse him, he should simply say, "I am fasting."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 7340