Qur'an, Al-Baqara, Surah 2:183-84
O you who believe! Fasting is ordained for you, as it was ordained for those before you, that you may attain piety. Fast a certain number of days, but those among you who are sick or traveling may fast the same number of days at a later time. But for those who are able to fast and do not, there is still redemption in the feeding of a needy person. However, for those who are given to increased virtue on their own accord, it is more valuable for them, though fasting is far better for you, if you only knew.