Sahih Muslim Book 6, Hadith Number 2563.

Chapter : When an observer of fast is invited to a feast or someone fights with him, he should say: “I am fasting”.

Abu Huraira (Allah be pleased with him) reported: When any one of you gets up in the morning in the state of fasting, he should neither use obscene language nor do any act of ignorance. And if anyone slanders him or quarrels with him, he should say: “I am fasting, I am fasting.”