Surat Al-Baqara (The Cow) - البقرة

أَيَّامًا مَّعْدُودَٰتٍ فَمَن كَانَ مِنكُم مَّرِيضًا أَوْ عَلَىٰ سَفَرٍ فَعِدَّةٌ مِّنْ أَيَّامٍ أُخَرَ وَعَلَى ٱلَّذِينَ يُطِيقُونَهُۥ فِدْيَةٌ طَعَامُ مِسْكِينٍ فَمَن تَطَوَّعَ خَيْرًا فَهُوَ خَيْرٌ لَّهُۥ وَأَن تَصُومُوا۟ خَيْرٌ لَّكُمْ إِن كُنتُمْ تَعْلَمُونَ ﴿١٨٤﴾

For a certain number of days; but whoever among you is sick or on a journey, then (he shall fast) a (like) number of other days; and those who are not able to do it may effect a redemption by feeding a poor man; so whoever does good spontaneously it is better for him; and that you fast is better for you if you know.