Sahih Bukhari Volume 003, Book 031, Hadith Number 128.
Narated By Abu Huraira : Allah's Apostle said, "Allah said, 'All the deeds of Adam's sons (people) are for them, except fasting which is for Me, and I will give the reward for it.' Fasting is a shield or protection from the fire and from committing sins. If one of you is fasting, he should avoid sexual relation with his wife and quarreling, and if somebody should fight or quarrel with him, he should say, 'I am fasting.' By Him in Whose Hands my soul is' The unpleasant smell coming out from the mouth of a fasting person is better in the sight of Allah than the smell of musk. There are two pleasures for the fasting person, one at the time of breaking his fast, and the other at the time when he will meet his Lord; then he will be pleased because of his fasting."