Qur'an, Al-Baqara, Surah 2:184
(Fasting) for a fixed number of days; but if any of you is ill, or on a journey, the prescribed number (Should be made up) from days later. For those who can do it (With hardship), is a ransom, the feeding of one that is indigent. But he that will give more, of his own free will, it is better for him. And it is better for you that ye fast, if ye only knew.