Abdullah bin Abbas (may Allah be pleased with them both) says that a woman came to the Prophet (peace and blessings be upon him) and said: My mother has died and she owed a month of fasts. He (peace and blessings be upon him) said: "What do you think, if she had owed a debt, would you have paid it?" She said: Yes. He (peace and blessings be upon him) said: "The debt of Allah is more deserving of being paid."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8223