Abu Huraira related that Rasulullah said: If anyone omits his fast even for one day in Ramadan without a concession or without being ill, then if he were to fast for the rest of his life he could not make up for it (Bukhari).
Abu Huraira related that the Prophet said: Allah the Majestic and Exalted said: "Every deed of man will receive ten to 700 times reward, except Siyam (fasting), for it is for Me and I shall reward it (as I like). There are two occasions of joy for one who fasts: one when he breaks the fast and the other when he will meet his Lord" (Muslim).
Abu Huraira related that Rasulullah said: Many people who fast get nothing from their fast except hunger and thirst, and many people who pray at night get nothing from it except wakefulness (Darimi).
When to start fasting
Ibn Umar related that the Prophet said: Do not start fasting unless you see the new moon, and do not end fasting until you see it. If the weather is cloudy then calculate when it should appear (Bukhari, Muslim).
The Suhoor meal (which is eaten before dawn)
Anas related that Rasulullah said: Take the Suhoor meal, for there is blessing in it (Bukhari, Muslim).
Breaking your fast
Salman ibn Amir Dhabi related that the Prophet said: Break your fast with dates, or else with water, for it is pure (Abu Dawud, Tirmidhi).
Fasting during a journey
Aisha related that Rasulullah was asked whether one should fast when on a journey, and he replied: Fast if you like, or postpone it if you like (Bukhari, Muslim).
Behavior while fasting
Abu Huraira related that the Prophet said: If a person does not avoid false talk and false conduct during Siyam, then Allah does not care if he abstains from food and drink (Bukhari, Muslim).
Forgetfully eating or drinking while fasting
Abu Huraira related that Rasulullah said: If anyone forgets that he is fasting and eats or drinks he should complete his Siyam, for it is Allah who has fed him and given him drink (Bukhari, Muslim).
Providing for those who are breaking the fast
Zaid ibn Khalid Juhni related that the Prophet said: He who provides for the breaking of the Siyam of another person earns the same merit as the one who was observing Siyam diminishing in any way the reward of the latter (Tirmidhi).
Aisha related that the Prophet said: Look for Lailat al-Qadr on an odd-numbered night during the last ten nights of Ramadan (Bukhari).
Anas ibn Malik related that Rasulullah said: When Lailat al-Qadr comes Gabriel descends with a company of angels who ask for blessings on everyone who is remembering Allah, whether they are sitting or standing (Baihaqi).