The Statement of Allah "[Observing Saum (fasts)] for a fixed number of days but if any of you is ill, or on a journey, the same number (should be made up) from other days. And as for those who can fast with difficulty (e.g., an old man, etc.) they have (a choice, either to fast or) to feed a Miskin (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast is better for you, if only you know." (V.2:184)
Reference
Sahih al-Bukhari 4505
In-book reference
Book 65, Hadith 32
USC-MSA web (English)
Vol. 6, Book 60, Hadith 32
Arabic Text
حَدَّثَنِي إِسْحَاقُ، أَخْبَرَنَا رَوْحٌ، حَدَّثَنَا زَكَرِيَّاءُ بْنُ إِسْحَاقَ، حَدَّثَنَا عَمْرُو بْنُ دِينَارٍ، عَنْ عَطَاءٍ، سَمِعَ ابْنَ عَبَّاسٍ، يَقْرَأُ {وَعَلَى الَّذِينَ يُطَوَّقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ }. قَالَ ابْنُ عَبَّاسٍ لَيْسَتْ بِمَنْسُوخَةٍ، هُوَ الشَّيْخُ الْكَبِيرُ وَالْمَرْأَةُ الْكَبِيرَةُ لاَ يَسْتَطِيعَانِ أَنْ يَصُومَا، فَلْيُطْعِمَانِ مَكَانَ كُلِّ يَوْمٍ مِسْكِينًا.