Imam Malik (may Allah have mercy on him) was informed that Sayyiduna Anas bin Malik (may Allah be pleased with him) became old to the extent that he could not fast, so he would give fidya (compensation).
Imam Malik, may Allah have mercy on him, said: In my view, paying fidya is not obligatory except for the one who has the ability to pay it; for such a person, it is better to give it. Whoever gives fidya should give one mudd of food for each fast, according to the mudd of the Messenger of Allah, peace and blessings be upon him.
Hadith Referenceموطا امام مالك رواية يحييٰ / كتاب الصيام / 627