Abdullah bin Abbas (may Allah be pleased with them both) said that a woman came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah! My sister has died and she owed two consecutive months of fasting. He (peace and blessings be upon him) said: "If your sister had owed a debt, would you have paid it?" She said: Yes. He (peace and blessings be upon him) said: "Then the right of Allah has a greater right to be fulfilled."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8225