Why do we fast?
It is from the nature of humans to ask and to wonder why they do things. Muslims do things to please Allah, and they can please Allah only by obeying Him and practicing His religion. Thus, in answer to the question some ignorants may ask "Why do we fast ?" we respond that it is a commandment and a clear order from Allah upon us and we have no other choice by to obey Allah and Please Him out of Love and Fear at the same time.

Allah says in the meaning of : "O you who believe, fasting has been prescribed for you as it has been prescribed for people before you so that you will (learn how to) attain Taqwa" (Qur'an, al-Baqarah, 2:183)
Ibn 'Umar reports that Allah's Messenger said: Islam is based on (the following) five (principles):
1. To testify that none has the right to be worshiped but Allah and Muhammad is Allah's Messenger.
2. To offer the (compulsory congregational) prayers dutifully and perfectly.
3. To pay Zakat (i.e. obligatory charity) .
4. To perform Hajj. (i.e. Pilgrimage to Mecca)
5. To observe fast during the month of Ramadan. [Bukhari]

Abu Huraira narrates that one day while Allah's Messenger was sitting with the people, a man came to him walking and said, "O Allah's Messenger. What is Belief?" The Prophet said, "Belief is to believe in Allah, His Angels, His Books, His Apostles, and the meeting with Him, and to believe in the Resurrection." The man asked, "O Allah's Apostle What is Islam?" The Prophet replied, "Islam is to worship Allah and not worship anything besides Him, to offer prayers perfectly, to pay the (compulsory) charity i.e. Zakat and to fast the month of Ramadan." [The narration of Muslim has Hajj as well]. The man again asked, "O Allah's Apostle What is Ihsan (i.e.. perfection or Benevolence)? The Prophet said, "Ihsan is to worship Allah as if you see Him, and if you do not achieve this state of devotion, then (take it for granted that) Allah sees you." .... Then the man left. The Prophet said, "Call him back to me." They went to call him back but could not see him. The Prophet said, "That was Jibreel (Gabriel) who came to teach the people their religion." [Bukhari]. (Note that there are other narrations of this hadith, I took here one from Bukhari)

Since Islam means submission to Allah, we have no other choice but to submit ourselves to Allah and obey His commandments. It is from the mercy of Allah towards us that while He prescribed on us fasting He also showed us the greatness of fasting in this world and in the hereafter to make it beloved to us.

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