Qur'an 2:184

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

“[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] - then an equal number of other 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 good [i.e., excess] - it is better for him. But to fast is best for you, if you only knew.”

Al-Baqara 2:184 · Saheeh International