Yahya related to me from Malik that he had heard that Anas ibnMalik used to pay fidya when he had grown old and could no longermanage to do the fast.Malik said, "I do not consider that todo so is obligatory, but what I like most is that a man does the fastwhen he is strong enough. Whoever pays compensation gives one mudd offood in place of every day, using the mudd of the Prophet, may Allahbless him and grant him peace."
USC-MSA web (English) reference: Book 18, Hadith 51
Arabic reference: Book 18, Hadith 684