Maliks Muwatta Book 18, Hadith Number 53.

Section : The Fidya (Compensation) of a Man who Breaks the Fast in Ramadan from Weakness.

Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasim that his father used to say, “If someone has to make up for days not fasted in Ramadan and does not do them before the next Ramadan comes although he is strong enough to do so, he should feed a poor man with a mudd of wheat for every day that he has missed, and he has to fast the days he owes as well.”

Yahya related to me from Malik that he had heard the same thing from Said ibn Jubayr.