The Pillars of Fasting

1. An-Niyyah (Intention): It is the resolve of the heart to fast in carrying out the
command of Allah the Mighty and Majestic, or to draw near to Him. This is
due to the statement of the Prophet :
”Deeds are judged by their intentions.” (Al-Bukhari)
If the fast is an obligatory fast, then the intention must be made during the
(previous) night, before Fajr time (the first crack of dawn). This is due to the
Prophet’s statement:
”One who does not plan (intend) to fast during the night, there is no
fast for him.” (At-Tirmidhi)
If it is a non-obligatory fast, it is correct (acceptable) even if the intention is
made after the entry of Al-Fajr (The first crack of dawn) and the progression
of daytime, as long as he has not eaten anything. This is due to the statement
of Aishah, may Allah be pleased with her:
The Messenger of Allah entered upon me one day and said:
”Do you have anything (to eat)?”
We said, “No”, He said: ”Then verily I am fasting.” (Muslim)

2. Al-Imsak (Refraining): This is to refrain from those things that break the fast,
such as eating, drinking and sexual intercourse.

3. Az-Zaman (The Time): This mean the daytime, which is from the entry of Fajr
(the first crack of dawn on the horizon) until the setting of the sun. So if a man
fasted during the night and did not fast during the day, his fast will never be
correct (i.e. acceptable). This is due to the statement of Allah the Almighty:

”And complete the fast until the night.” (2:187)

Taken from http://www.ilmpath.com/free/Ramadan-ebook.pdf

Sahih Al - Bukhari Hadith
Volume 2, Book 21, Number 231:
Narrated Abdullah bin 'Amr bin Al-'As:

Allah's Apostle (sallalahu alayhi wa salam) told me, "The most beloved prayer to Allah is that of David (Daud) and the most beloved fasts to Allah are those of David (Daud). He used to sleep for half of the night and then pray for one third of the night and again sleep for its sixth part and used to fast on alternate days."

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Sahih Al - Bukhari Hadith
Volume 2, Book 21, Number 247:
Narrated Al-Aswad:

I asked 'Aisha "How is the night prayer of the Prophet (sallalahu alayhi wa salam)?" She replied, "He used to sleep early at night, and get up in its last part to pray, and then return to his bed. When the Muadh-dhin pronounced the Adhan, he would get up. If he was in need of a bath he would take it; otherwise he would perform ablution and then go out (for the prayer)."