The Commander of the Faithful [ALI (A.S.)] says that a party of the Jews came to the Prophet (S.A.) The most learned among them put a few questions, one of which was: 'What is the good to your people in that Allah has enjoined fast on them and that, too only for thirty days (excluding nights) whereas the other people had to fast for more than thirty days?'
The Prophet (S.A.) replied: "When Adam ate of that tree the fruit remained ill his stomach for thirty days. So Allah enjoined upon his progeny to remain hungry and thirsty for that period, and the permission to eat in the night is Allah's grace. Adam also had to fast for so many days and Allah enjoined those fasts upon my people."
Then the Prophet (S.A.) recited this verse: Fasting has been ordained to you as it was ordained to those before you so that you may guard yourself against evil (and that too) for counted days'.
The Jew savant said, Mohammad, You are right. Now tell me what is the reward for a person who observes those fasts." The Prophet replied: " Allah will characterize the believer who keeps the fasts in Ramazan to please Allah, with seven traits:
(i) If he has partaken of the forbidden food, it will be excreted after being digested.
(ii) He will not be deprived of the mercy of Allah.
(iii) He will share the compensation of his father's (Adam) fault,
(iv) Allah will grant him relief in his agony of death.
(v) The observer of fast will not feel hungry and thirsty On the Day of Judgment.
(vi) Allah will grant him deliverance from Hell fire on the day of Resurrection.
(vii) He will satisfy him with the sacred diet of Paradise.
The Jew said:
"Mohammad, you are right."
source : rafed.net