Allah says in the Qur'an:

"O you who believe!  Fasting has been prescribed for you, as it was prescribed for those before you, so that you may guard yourselves (against evil), for a counted number of days; but whoso among you be sick or on a journey then (he shall fast) that number of other days; and those who are with difficulty able to do so, on them is a redemption by feeding a poor man, and whoso on his own accord performs good, it is better for him, and that you fast is better for you if you know;  The month of Ramadan, in which was sent down the Qur'an, a guidance for the people, and clear evidence of guidance and discrimination (between right and wrong); so whosoever of you witnesses the month, he shall fast therein; and whosoever is sick or on a journey (he shall fast) the same number of other days;  Allah desires ease for you and he desires not hardship for you; and so that you may complete the (prescribed) number, and that you may glorify Allah and his guiding you, and so that you may be thankful to Him."  (2:183-185)

Ja'far ibn Muhammad As-Sadiq (peace be upon him) related that the Messenger of Allah (may Allah bless him and grant him peace) said, "Fasting is a shield which protects [one] from the ills of this world and is a barrier against the torment of the next.  If you fast, then make an intention of restraining the passions of the self, of removing any desire for the way of Satan, and bring yourself to the station of the contented, neither craving food nor drink, waiting expectantly every moment to be cured from the illness of wrong actions.  Moreover, when you fast you should purify your inward self from any impurity, negligence or darkness which cuts you off from sincere devotion to the essence of Allah."  (Qurrat al-'Uyun)