It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings of Allah be upon him) said: When any one of you is fasting on any day, he should neither speak indecently nor act ignorantly. If someone behaves ignorantly towards him, he should say, "I am fasting."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9191