Maliks Muwatta Book 18, Hadith Number 57.

Yahya related to me from Malik from Abu’z Zinad from al-A’raj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “Fasting is a protection for you, so when you are fasting, do not behave obscenely or foolishly, and if any one argues with you or abuses you, say, ‘I am fasting. I am fasting.'”