Sahih al-Bukhari 4505
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
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)
Narrated 'Ata: That he heard Ibn `Abbas reciting the Divine Verse:-- "And for those who can fast they had a choice either fast, or feed a poor for every day.." (2:184) Ibn `Abbas said, "This Verse is not abrogated, but it is meant for old men and old women who have no strength to fast, so they should feed one poor person for each day of fasting (instead of fasting).