Musnad Ahmad 30:3504

Rawh narrated to us, Isra'il narrated to 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, nor act ignorantly, nor harm anyone. If someone acts ignorantly toward him or harms him, let him say: 'I am fasting.'"