Musnad Ahmad 30:2069

Yahya ibn Ishaq narrated to us, he said: Abu Bakr ibn Ayyash informed us, from Abu Hasin, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah, may Allah bless him and grant him peace, 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.'"