Nafi' (may Allah have mercy on him) says that Ibn Umar (may Allah be pleased with them both) said: The Prophet (peace and blessings be upon him) was asked about a person who died while owing a month of fasts, and he (peace and blessings be upon him) said: "A poor person should be fed on his behalf for each day."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8218