narrated abu huraira (radi-allahu 'anhu):

allah's apostle (sallallahu 'alaihi wa sallam) said, "fasting is a shield (or a screen or a shelter). so, the person observing fasting should avoid sexual relation with his wife and should not behave foolishly and impudently, and if somebody fights with him or abuses him, he should tell him twice, 'i am fasting." the prophet (sallallahu 'alaihi wa sallam) added, "by him in whose hands my soul is, the smell coming out from the mouth of a fasting person is better in the sight of allah than the smell of musk. (allah says about the fasting person), 'he has left his food, drink and desires for my sake. the fast is for me. so i will reward (the fasting person) for it and the reward of good deeds is multiplied ten times."

bukhari vol. 3 : no. 118