Al-Baqarah 2:184

أَيَّامٗا مَّعۡدُودَٰتٖۚ فَمَن كَانَ مِنكُم مَّرِيضًا أَوۡ عَلَىٰ سَفَرٖ فَعِدَّةٞ مِّنۡ أَيَّامٍ أُخَرَۚ وَعَلَى ٱلَّذِينَ يُطِيقُونَهُۥ فِدۡيَةٞ طَعَامُ مِسۡكِينٖۖ فَمَن تَطَوَّعَ خَيۡرٗا فَهُوَ خَيۡرٞ لَّهُۥۚ وَأَن تَصُومُواْ خَيۡرٞ لَّكُمۡ إِن كُنتُمۡ تَعۡلَمُونَ 184

[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess - it is better for him. But to fast is best for you, if you only knew