sawm is wajib (compulsory) on muslims, according to the quran and sunnah [tradition of the prophet (peace be upon him)], during the ninth 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 it…"

[noble quran 2:185]

ibn ‘umar reported that the prophet (peace be upon him) stated, "islam is built on five (pillars): ….fasting in ramadan."

talhah ibn ‘ubaydillah 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." he (peace be upon him) said, "the month of ramadan." so he asked, "is there anything other than that on me?" he (peace be upon him) said, "no, except what you do voluntarily." he then said, "tell me what allah has made fard on me concerning zakah, "so he (peace be upon him) informed him of the rites of islam. he then said, "by he who ennobled you, i won't voluntarily do anything (extra) nor will i decrease in any way what allah has made obligatory on me." the prophet (peace be upon him) then said, "he will succeed if he spoke the truth (or, he will enter paradise if he was truthful).