In the Chapter Al-Baqarah of the Quran, there are five verses that deal with fasting in the month of Ramadan. (2:183-187). In the book of Hadith Mishkat al-Masabih, seventy-three sayings of the Prophet have been quoted with regard to fasting. Detailed provisions regarding Ramadan can be found in the books of fiqh.
In a Hadith, the Prophet of Islam has said: "Every deed in Islam is rewarded from ten to seven hundred times. But God says, 'Fasting is for me, and I will give unlimited reward to one who fasts.'" (Sahih Muslim, Hadith No. 1151)
The question is: what is the importance of fasting such that one who fasts shall be given unlimited reward? The above Hadith gives the reason. It says that one who fasts gives up food and water for the sake of God.
Here, "for the sake of God" is not an ambiguous phrase. It means that a person who fasts, prepares himself for the mission of God. Due to this high purpose, he will be given the highest reward.
In the month of Ramadan, it is required that every believer must go through the Quran and find out the message of the Quran and adopt the spreading of this message as his mission. So, every year, one month is meant for preparation and eleven months for spreading the message of the Quran to all mankind.
Spreading the Quranic message is a divine mission. According to the Quran, it is akin to providing support to God. Due to this speciality, the Prophet of Islam informed that fasting is preparing oneself for the sake of the divine mission. This is the greatest mission, so one who devotes himself for it is liable for the greatest reward.
The Quran says: "The month of Ramadan is the month when the Quran was sent down as guidance for mankind with clear proofs of guidance and the criterion by which to distinguish right from wrong. Therefore, whoever of you is present in that month, let him fast." (2:185)
In fact, the month of Ramadan is the month of etikaf (going into seclusion). Minimum etikaf is for ten days and maximum etikaf is for one month. Fasting means staying away from all kinds of distractions, including abstaining from food and water for a temporary period, thus freeing oneself from all other activities and totally engaging in the study of the Quran.
If you go through the Quran, you will find that the Quran is a book of mission. And, fasting in the month of Ramadan means saving yourself from all kinds of distractions, thus saving time for the study of the Quran. In this sense, the month of Ramadan is the month of discovery of the Quran. The month of Ramadan is to discover the Quran and then to try to spread its message all over the world.
One who observes fasting in this sense and prepares himself for the mission of the Quran and dedicates himself for this dawah mission, is promised unlimited reward.