Surah Al-Bakarah, Ayah-185, Holy Quran

The month of Ramadan in which Quran was sent down, the direction for the people and guidance and clear
signs of judgement; that whoso of you finds this month, necessarily, he should fast in it, and whoso is ill or on
a journey then same number of fasting in other days; Allah desires facility for you and desires not hardship and
for this, that you should complete the counting and speak of the Greatness of Allah, as He guided you and so
that you may be grateful...

Surah Al-Bakarah, Ayah-184, Holy Quran

[Observing Saum (Fast)] For A Fixed Number Of Days, But If Any Of You is ill or, On A Journey, The Same Number Should Be Made Up From The Other Days. And As Of Those Who Can Fast With Difficulty (eg. An Old Man), They Have A Choice Either To Fast or, Feed A Miskin (Poor Person) For Everyday. But, Whoever Does Good Of His Own Accord, it is Better For Him. And That You Fast is Better For You if Only You Know...