Surah 2, Verse 185

Ramadan is the month in which the Qur´an was revealed as guidance to man and clear proof of the guidance, and criterion (of falsehood and truth). So when you see the new moon you should fast the whole month; but a person who is ill or travelling (and fails to do so) should fast on other days, as God wishes ease and not hardship for you, so that you complete the (fixed) number (of fasts), and give glory to God for the guidance, and be grateful.

— Quran 2:185

Quote from Quran 2:185