Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that a man came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, my mother vowed a month of fasting, and she has died. The Prophet (peace and blessings be upon him) said: "Tell me, if she had owed a debt, would you have paid it off?" He said: Yes. He (peace and blessings be upon him) said: "Then the debt of Allah is more deserving of being paid."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12279