Musnad Ahmad 30:2868

Ishaq narrated to us, he said: Malik narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, fasting is a shield. So if one of you is fasting, let him not use obscene speech, and let him not act ignorantly. If a person insults him or fights him, let him say: 'I am fasting.'"