Musnad Ahmad 30:2817

'Abd al-Rahman narrated to us, Sulaym narrated to us, from Sa'id, from Abu Hurayrah, from the Prophet (peace be upon him). And Bahz said: Sulaym ibn Hayyan narrated to me, he said: Sa'id narrated to us, from Abu Hurayrah, from the Prophet (peace be upon him), who said: "Fasting is a shield. If one of you is fasting on a day, he should not use obscene speech, nor should he act ignorantly. If someone insults him—or if a man insults him—let him say: 'I am fasting.'" Bahz said: "If a man insults him or fights him, let him say: 'I am fasting.'" And likewise, 'Affan said: "Or fights him."