Muwatta Imam Malik Book of Fasting, Hadith 51

Yahya related to me from Malik that he had heard that Anas ibn Malik used to pay fidya when he had grown old and could no longer manage to do the fast.
Malik said, "I do not consider that to do so is obligatory, but what I like most is that a man does the fast when he is strong enough. Whoever pays compensation gives one mudd of food in place of every day, using the mudd of the Prophet, may Allah bless him and grant him peace."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ أَنَسَ بْنَ مَالِكٍ، كَبِرَ حَتَّى كَانَ لاَ يَقْدِرُ عَلَى الصِّيَامِ فَكَانَ يَفْتَدِي . قَالَ مَالِكٌ وَلاَ أَرَى ذَلِكَ وَاجِبًا وَأَحَبُّ إِلَىَّ أَنْ يَفْعَلَهُ إِذَا كَانَ قَوِيًّا عَلَيْهِ فَمَنْ فَدَى فَإِنَّمَا يُطْعِمُ مَكَانَ كُلِّ يَوْمٍ مُدًّا بِمُدِّ النَّبِيِّ ﷺ .