Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2563
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."