Musnad Ahmad 30:3421

Yazid narrated to us, Salim ibn Hayyan informed us, Sa'id narrated to us, he said: I heard Abu Hurairah say: The Messenger of Allah, may Allah bless him and grant him peace, said: "Fasting is a shield. So if one of you is fasting on a day, he should not use obscene speech nor act ignorantly. And if a man insults him or fights him, let him say: 'I am fasting.'"