Narrated by Abu Huraira (Radhiallaho anho):
Allah's Messenger (sallallaahu 'alaihi wasallam) 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."

Bukhari Vol. 3 : No. 128