Nafi' narrates that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) used to say: Whoever did not fast some days of Ramadan because of illness and then died before making them up, then on his behalf one mudd of food should be given to a poor person in place of each day, for all those days. If the next Ramadan reaches him before he has fasted them, and he is able to fast the one that has come, then for the days that have passed he should give a poor person one mudd of wheat (for each day), and fast the Ramadan that is upon him.
This is the correct version, mawquf (a saying of Ibn Umar); Muhammad bin Abd al-Rahman bin Abi Layla narrated it from Nafi' and erred in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8216