It was narrated from 'Ata from Ibn 'Abbas:

concerning this verse "And as for those who can fast with difficulty, (a choice either to fast or) to feed a Miskin (poor person) (for every day)." That for those who can fast with difficulty means they find it hard; to feed a Miskin means feeding one poor person for each day. But whoever does good of his own accord means feeding another poor person. This is not abrogated, and it is bette for him. And: that "you fast is better for you" means there is no concession regarding this except for those who are not able to fast, or who are incurably sick.

أَخْبَرَنَا مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا يَزِيدُ، قَالَ أَنْبَأَنَا وَرْقَاءُ، عَنْ عَمْرِو بْنِ دِينَارٍ، عَنْ عَطَاءٍ، عَنِ ابْنِ عَبَّاسٍ، فِي قَوْلِهِ عَزَّ وَجَلَّ ‏{‏ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ ‏}‏ يُطِيقُونَهُ يُكَلَّفُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ وَاحِدٍ ‏{‏ فَمَنْ تَطَوَّعَ خَيْرًا ‏}‏ طَعَامُ مِسْكِينٍ آخَرَ لَيْسَتْ بِمَنْسُوخَةٍ ‏{‏ فَهُوَ خَيْرٌ لَهُ وَأَنْ تَصُومُوا خَيْرٌ لَكُمْ ‏}‏ لاَ يُرَخَّصُ فِي هَذَا إِلاَّ لِلَّذِي لاَ يُطِيقُ الصِّيَامَ أَوْ مَرِيضٍ لاَ يُشْفَى ‏.‏