Musnad Abu Ya'la al-Mawsili 159:18

Abu Khaythama narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah: If one of you wakes up 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, I am fasting."