Musnad Ahmad 30:374

Yazid narrated to us, Muhammad informed us, from Musa ibn Yasar, from Abu Hurairah, and from Abu al-Zinad, from al-A'raj, from Abu Hurairah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Fasting is a shield. If one of you is fasting on a day, he should not use obscene language, nor should he act ignorantly. If a person fights him or insults him, let him say: 'I am fasting, I am fasting.'"