Rasulullah (Sallallahu alaihi wasallam) states: I swore by the One (Allah) in Whose Hand is the soul of Muhammad, the breath of the faster is sweeter to Allah on the Day of Judgment than the scent of musk. The faster experiences enjoyment twice: he is pleased when he breaks his fast, and he is pleased when he meets his Maker" (Muslim)
The above Hadith also gives the faster good news regarding the changing breath of the faster, which is sweeter to Allah (SWT) than musk. The changing breath of the mouth is a vivid physical testimony of this discipline. As much as we hate bad breath, in fasting, it is a good thing, for it is caused by the coating which appears on the upper surface of the tongue soon after the commencement of the fast. After desiring food, the body begins to digest such waste material and deposits of fat as are available to it. This coating on the tongue is outward proof that inner elimination is in progress. As soon as the digestive organs have been purified, the mouth returns to normal.
In a hadith related by Abdullah Bin Omar, the Prophet said: "Fasting (Siyaam) and the Book of Allah (Al-Qur'an) will intercede for the servant on the Day of Judgment. The fasting will say: 'O, Allah, I denied him nourishment and sex during the daytime. Let me intercede on his behalf, and Al-Qur'an will say: 'I denied him sleep during the night time; so let me intercede on his behalf.' So Allah will allow them to intercede on his behalf." (Ahmed)
This hadith indicates on the Day of Judgment, when the events will be overwhelming and the outcome uncertain, the fast and the Book will intercede for the servants of Allah. As every second of daytime devotion and every letter of Al-Qur'an voices, their pleasure openly with you during the witnessing and interviewing, bear in mind that Shafa'ah (intercession), is an extreme privilege and a rare commodity. Even the average prophet will not attempt to intercede for anyone on this day.
Shal bin Sa'ad reported the Messenger of Allah (saas) said: "Paradise (Jannah) has a door called Rayyan (the quencher), which is preserved for those who observed fasting on the Day of Judgment. It will be announced, 'Where are those who observed fasting?' The door, Rayyan, will not be closed until the last one of them enters." (Bukhari and Muslim)
This is what the pleasure of Allah is all about. The person who observes fasting becomes an elite in the hour when every other person is busy turning the pages of their books of deeds. This is the time you are pulled away through the Rayyan Gate to Paradise.
Abu Sa'eed Al-Khudree related the Messenger of Allah (saas) said: "If a servant of Allah fasts a day for the pleasure of Allah (SWT), He will distance his face from the hellfire, in the equivalent of that day, which, in the sight of Allah, will take seventy years to cover." (Bukhari/Muslim)
The Hereafter is real, and rewarding evil in kind is also real; but as a reward to those who observe the fast, Allah (SWT) will distance them from the ultimate punishment. These four reports underscore the spiritual as well as the physical value of Siyaam, fasting.