Allah has enjoined upon the Muslims fasting the month of Ramadaan, and He has promised a great reward to those who fast. Because the virtue of fasting is so great, Allah has not specified the reward for it, rather He said – in a hadeeth qudsi –
The virtues of the month of Ramadaan are many. For example, that which Allah has promised those who fast, namely the gate of al-Rayyaan. This is the name which was narrated in the hadeeth whose authenticity is agreed upon. Sahl (may Allaah be pleased with him) said that the Prophet (peace and blessings of Allaah be upon him) said:
“In Paradise there is a gate called al-Rayyaan, through which those who used to fast will enter on the Day of Resurrection, and no one but they will enter it. It will be said, ‘Where are those who fasted?’ They will get up, and none will enter it but them. When they have entered, it will be locked, and no one else will enter.” (al-Bukhaari, 1763; Muslim, 1947).
Among the haadeeth which explain the reward for fasting are the following:
It was narrated from Abu Salamah that Abu Hurayrah said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Whoever fasts Ramadaan out of faith and seeking reward, his previous sins will be forgiven.’” (Narrated by al-Bukhaari, al-Eemaan, 37).
It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Allaah says: “Every deed of the son of Adam is for him, except for fasting; it is for Me and I shall reward for it.” Fasting is a shield and when one of you is fasting he avoid sexual relations with his wife and quarreling. If somebody should fight or quarrel with him, he should say, ‘I am fasting.’ By the One in Whose hand is my soul, the unpleasant smell coming out from the mouth of a fasting person is better in the sight of Allaah than the smell of musk. The fasting person will have two moments of joy: one when he breaks his fast, and the other when he meets his Lord; then he will be pleased because of his fasting.’”(Narrated by al-Bukhaari, 1771)