Musnad Ahmad 30:3002

Asbat ibn Muhammad narrated to us, he said: Al-A’mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When it is a day on which one of you is fasting, 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.'"