“O you who believe! Fasting is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqun (the pious).
(Fasting) for a fixed number of days, but if any of you is ill or on a journey, the same number (should be made up) from other days. And as for those who can fast with difficulty, (i.e. an old man, etc.), they have (a choice either to fast or) to feed a poor person (for every day). But whoever does good of his own accord, it is better for him. And that you fast, it is better for you if only you know.” (2:183-84)
“The month of Ramadan in which was revealed the Quran, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan), he must fast that month, and whoever is ill or on a journey, the same number (of days which one did not fast must be made up) from other days.
Allah intends for you ease, and He does not want to make things difficult for you. (He wants that you) must complete the same number (of days), and that you must magnify Allah for having guided you so that you may be grateful to Him” (2: 185).
“It is made lawful for you to have sexual relations with your wives on the night of the fasts. They are garments for you and you are the same for them. Allah knows that you used to deceive yourselves, so He turned to you and forgave you. So now have sexual relations with them and seek that which Allah has ordained for you, and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your fast till the nightfall.
And do not have sexual relations with them (your wives) while you are in Itikaf in the mosques. These are the limits (set) by Allah, so approach them not. Thus does Allah make clear His signs to mankind that they may become Al Muttaqun (the pious)” (2:187).
“Verily! We have sent it (this Quran) down in the night of Al-Qadr.
And what will make you know what the night of Al-Qadr is?
The night of Al-Qadr is better than a thousand months
Therein descend the angels and the Ruh (Gabriel) by Allah’s Permission with all Decrees,
Peace! until the appearance of dawn (97:1-5)
THE HADITH ON FASTING/RAMADAN
Abu Hurairah, (May Allah be pleased with him) reported: Allah’s Messenger (PBUH) said: “When there comes the month of Ramadan, the gates of Paradise are opened, and the gates of Hell are locked and the devils are chained.” (MUSLIM)
Abu Huraira related that the Prophet (peace and blessings be upon him) said: Whoever fasts during Ramadan with faith and seeking his reward from Allah will have his past sins forgiven. Whoever prays during the nights in Ramadan with faith and seeking his reward from Allah will have his past sins forgiven. And he who passes Lailat al-Qadr in prayer with faith and seeking his reward from Allah will have his past sins forgiven (Bukhari, Muslim).
The Prophet peace be upon him said “Whoever does not give up false statements (i.e. telling lies), and evil deeds, and speaking bad words to others, Allah is not in need of his (fasting) leaving his food and drink.” [Bukhari]
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 Sa’id Al-Khudri (may Allah be pleased with him) reported: Allah’s Messenger (may peace be upon him) said: Any servant of Allah who observes one day of Fast for Allah’s sake, Allah would remove his face away from the Fire of Hell a distance equivalent to seventy years. (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) (ie. those who has less faith and doesnt give up forbidden acts like backbiting while fasting)
The Messenger of Allah (may Allah bless him and grant him peace) said: “There is a gate in the Garden called ar-Rayyan which those who fast will enter on the Day of Rising, and none but they will enter it.” It will be said, ‘Where are the fasters?’ They will stand up and none but they will enter it. When they have entered it, it will be closed and no one else will enter it.” [Al Bukhari Hadees # 1773] [Al Muslim Hadees #1954]
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).
It is narrated from Sahl bin Sad (radiyAllaahu anhu): Allah’s Apostle (salallaahu ‘alaihi wa salam) said: “The people will remain on the right path as long as they hasten the breaking of the fast.” [Bukhari: Book 3 Vol 31 #178; Muslim: Book 6 #2417]
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).
Bukhari,Volume 3, Book 32, Number 241: Narrated Aisha(ra): “With the start of the last ten days of Ramadan, the Prophet(PBUH) used to tighten his waist belt (i.e. work hard) and used to pray all the night, and used to keep his family awake for the prayers.”
Courtesy: http://soundvision.com/Info/ramadan/qhfast.asp and other Islamic books