Sunan of Abu Dawood Book 14, Hadith 2316

حَدَّثَنَا أَحْمَدُ بْنُ مُحَمَّدٍ، حَدَّثَنِي عَلِيُّ بْنُ حُسَيْنٍ، عَنْ أَبِيهِ، عَنْ يَزِيدَ النَّحْوِيِّ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، ‏{‏ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ ‏}‏ فَكَانَ مَنْ شَاءَ مِنْهُمْ أَنْ يَفْتَدِيَ بِطَعَامِ مِسْكِينٍ افْتَدَى وَتَمَّ لَهُ صَوْمُهُ فَقَالَ ‏{‏ فَمَنْ تَطَوَّعَ خَيْرًا فَهُوَ خَيْرٌ لَهُ وَأَنْ تَصُومُوا خَيْرٌ لَكُمْ ‏}‏ وَقَالَ ‏{‏ فَمَنْ شَهِدَ مِنْكُمُ الشَّهْرَ فَلْيَصُمْهُ وَمَنْ كَانَ مَرِيضًا أَوْ عَلَى سَفَرٍ فَعِدَّةٌ مِنْ أَيَّامٍ أُخَرَ ‏}‏ ‏.‏

Ibn ‘Abbas explain the Qur'anic verse "For those who can do it(with hardship) is a ransom, the feeding of one, that is indigent" said "If one of them wished to pay ransom by providing food to an indigent person he could pay ransom.. Thus, his fast was complete. Allaah, the Exalted pronounced "But he that will give more of his own free will, it is better for him". Again he pronounced "So every one of you who is present (at his home) during that month should spend it in fasting." But, if anyone is ill or on a journey the prescribed period (should be made up) by days later