Say: 'I Am Fasting.'

And from the way of Abu Hurayra, who said: 'The Messenger of Allah (PBUH) said: "Fasting is a shield. When anyone of you is fasting, he should neither behave in an obscene manner nor do any act of ignorance. If a man fights or abuses him, let him say: 'I am fasting.' "'

Related by Al-Rabii bin Habib (Al Jami' Al Sahih, Hadith No: 330)