Sahih al-Bukhari

Sahih al-Bukhari, Hadith 4505

• Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
• Chapter 25: 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)

باب قَوْلِهِ ‏{‏أَيَّامًا مَعْدُودَاتٍ فَمَنْ كَانَ مِنْكُمْ مَرِيضًا أَوْ عَلَى سَفَرٍ فَعِدَّةٌ مِنْ أَيَّامٍ أُخَرَ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ فَمَنْ تَطَوَّعَ خَيْرًا فَهُوَ خَيْرٌ لَهُ وَأَنْ تَصُومُوا خَيْرٌ لَكُمْ إِنْ كُنْتُمْ تَعْلَمُونَ‏}‏