Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that a man came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah! My mother has died and she owed a month of fasting; shall I make it up on her behalf? He (peace and blessings be upon him) said: "If your mother had owed a debt, would you have paid it?" He said: Yes. So he (peace and blessings be upon him) said: "Then the debt of Allah has a greater right to be paid."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8224