Musnad Ahmad 2004

It was narrated from Ibn "Abbas that a woman said: O Messenger of Allah, [my] mother owed the fasting of one month but she died; should I fast on her behalf? He said: “If your mother owed a debt, wouldn"t you pay it off?” She said: Yes. He said: "The debt of Allah, may He be glorified and exalted, is more deserving of being paid off.”