Islam is built on five (pillars)
It was narrated that Ibn ‘Umar (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Islam is built on five (pillars): bearing witness that there is no god except Allaah and that Muhammad is the Messenger of Allaah, establishing prayer, paying zakaah, Hajj and fasting Ramadaan.” Narrated by al-Bukhaari, 8; Muslim, 16.
This hadeeth indicates that it is obligatory to fast Ramadaan, and that it is one of the pillars and most important fundamentals of Islam. Allaah has enjoined it upon His slaves by His wisdom for great reasons; those who know them know them and those who do not do not.
The purpose of fasting is to rein in one’s desires and wean oneself from habits and control one's physical desires, so that one will be ready to start seeking that which will ultimately bring happiness, and purify the soul to live in Paradise forever. Hunger and thirst can reduce these desires and their overwhelming effects, and remind a person of the situation of the starving poor.
Fasting also reduces the influence of the Shaytaan, just as his food and drink are reduced. It restrains the faculties from following a person’s inclinations in a way that could harm him in this world and in the Hereafter.
So every faculty and every desire is restrained. Fasting is the rein of the pious, the shield of the fighter, the paradise of the righteous…
Fasting has a wonderful effect of protecting physical faculties and mental and spiritual energies, and protecting them from going to extremes in following inclinations that could adversely affect the health. Fasting protects the physical and spiritual well being and restores that which was damaged as a result of following whims and desires. So it is one of the best aids to attaining taqwa (piety).
From the words of Ibn al-Qayyim.
Fasting brings a great reward which, if the fasting person could imagine it, his heart would soar with joy, and he would wish that the whole year was Ramadaan. This was the understanding of the earliest generations of this ummah.
Fasting brings many health benefits and achieves the interests of both communities and individuals. And Allaah is All-Knowing, Most Generous.
The texts indicate that whoever does the duties and obligations of Islam, and avoids the things that are forbidden, will be one of the people of Paradise. Talhah ibn ‘Ubayd said: A man came to the Messenger of Allaah (peace and blessings of Allaah be upon him) and asked him about Islam. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “(You have to do) five prayers every day and night.” He said: “Do I have to do any others?’ He said: “No, not unless you do them voluntarily.”
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “And fast the month of Ramadaan.”
He said: “Do I have to do any more?” He said: “No, not unless you do it voluntarily.”
And the Messenger of Allaah (peace and blessings of Allaah be upon him) told him about zakaah and he said: “Do I have to do anything else?’ He said: “No, not unless you do it voluntarily.”
The man left, saying, “By Allaah, I will not any more or any less than that.”
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “He will prosper if he meant what he said.” Narrated by al-Bukhaari, 6956; Muslim, 11.
Praise be to Allaah who has prescribed the action and helped us to achieve it and has decreed reward for it, for He is the Most Forgiving.
See Ahaadeeth al-Siyaam by al-Fawzaan, p. 17