Definition of Siyam (Fasting): Siyam in Arabic means "to abstain from something". For example, Allah says about Mariam (Mary) in the Qur'an that she said [meaning]: "Verily!, I have vowed a fast to the Most Beneficent (Allah) so I shall not speak to any human this day "[Maryam 19:26]. In religion (Islam) Siyam means "abstaining from food, drink, and sexual intercourse from dawn until sunset with the sincere intention of doing so (for the sake of Allah). Therefore the pillars of fasting are :

1-The Intention (Niyya):

To sincerely fast for the sake of Allah. The intension need not be uttered. It is in reality an act of the heart which does not involve the tongue. It is a bid`ah [innovation] to do so. The fact that one knows he is fasting the next day is "intention". The fact of taking Suhoor is "intention. About sincerity in acts of worship Allah said in the Qur'an [in the meaning of]: "And they were ordained nothing else than to worship Allah, keeping religion pure for Him" [al-Bayyinah,

Also the Messenger of Allah (S) said : Actions are judged according to the intention behind them, and for everyone is what he intended" [Bukhari and Muslim]

The intention must be made before fajr and during every night of Ramadan and is valid if made in any part of the night.

Hafsah reported that the Prophet (S) said: "Whoever does not determine to fast before fajr will have no fast" (that is it won't be accepted) [an-Nasa'i, at-Tirmidhi, Abu Dawud, and others, Sahih].

2-Abstaining from the acts that break the fast:

Allah says in the Qur'an [in the meaning of]: "Eat and drink until the white thread becomes distinct to you from the black thread of the dawn. Then strictly observe the fast until nightfall." [al-Baqara, 2:187]

`Adiyy Ibn Hatim said "When the Ayah 'Eat and drink until the white thread becomes distinct to you ... was revealed, I took a black thread and a white thread and placed them underneath my pillow. During the night I looked at them to if I could distinguish between them. In the morning I went the Messenger of Allah and mentioned that to him and he said: 'It is the black of the night and the white of the day.' " [Bukhari and Muslim]

Therefore, the believer must fast from Fajr [dawn] to Maghrib [sunset].