Musnad Ahmad 30:940

Bahz narrated to us, Sulaym ibn Hayyan narrated to us, Sa'id narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fasting is a shield. So if one of you is fasting on a day, he should not use obscene speech nor act ignorantly. If a man insults him or fights him, let him say: 'I am fasting.'"