Musnad Ahmad 30:2813

'Abd al-Rahman narrated to us, he said: Sufyan narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "When it is the day of fasting for one of you, let him not use obscene speech, and let him not act ignorantly. If someone acts ignorantly toward him, let him say: 'I am a fasting person.'"