Musnad Ahmad 30:722

Muhammad ibn Bakr narrated to us, Isra'il informed us, from Abu Hasin, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) 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.'"