Musnad Ahmad 30:222

Sufyan narrated to us, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, in a narration—he said once: He attributes it to the Prophet, may Allah bless him and grant him peace—: "When 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.'"