FASTING (SAWM OR SIYAM)
The literal meaning of Sawm is to 'Restrain oneself from something'. For example, Allah tells us in al-Quran that Maryam said:
"Surely I have vowed to Ar-Rahman (The Most Merciful, Allah) to Fast (sawm)." [Holy Quran-19:26]
(The Fast here means to be silent, that is to abstain from speaking.)
Note: Islamically, 'Sawm means to abstain from Food, Drink and Sexual relation between dawn and sunset'.
CLASSIFICATION OF SAWM
Sawm is (Wajib) compulsory on Muslims, according to the Quran and Sunnah [Tradition of the Prophet (Peace be upon him)], during the 9th Month of the Hijri calendar, called Ramadan every year.
Allah said in the Quran:
"The Month of Ramadan: in it was sent down the Quran, Guidance to Mankind and a Self-evident proof of that Guidance and the standard for distinguishing between Right and Wrong. Whosoever of you witnesses the month should fast." [Holy Quran-2:185]
Ibn Umar reported that the Prophet (Peace be upon him) stated: "Islam is built on 5 (Pillars) ... Fasting in Ramadan is 3rd Pillar."
Talha ibn Ubaydullah reported that a man with his hair in disarray came to the Prophet (peace be upon him) and said,
"O Allah's Messenger, inform me of what Allah has made Fard (Obligatory) on me concerning Fasting?" Prophet (Peace be upon him) said: "The Fasting of whole month of Ramadan."