Musnad Ahmad 30:1553

Aswad said: With this chain of narration, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When it is the day of fasting for any one of you, let him not use obscene speech, nor act wickedly, nor behave ignorantly. If someone acts ignorantly toward him, let him say: 'I am a fasting person.'"