The ill and traveller are told to make up the same number of missed days later.
أَيَّامࣰ ا مَّعۡدُودَٰتࣲۚ فَمَن كَانَ مِنكُم مَّرِيضًا أَوۡ عَلَىٰ سَفَرࣲ فَعِدَّةࣱ مِّنۡ أَيَّامٍ أُخَرَۚ وَعَلَى ٱلَّذِينَ يُطِيقُونَهُۥ فِدۡيَةࣱ طَعَامُ مِسۡكِينࣲۖ فَمَن تَطَوَّعَ خَيۡرࣰ ا فَهُوَ خَيۡرࣱ لَّهُۥۚ وَأَن تَصُومُواْ خَيۡرࣱ لَّكُمۡ إِن كُنتُمۡ تَعۡلَمُونَ ١٨٤
[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