Musnad Ahmad 25:133

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Muslim al-Batin, from Sa'id ibn Jubayr, from Ibn Abbas, who said: A woman came to the Prophet, may God bless him and grant him peace, and said: "O Messenger of God, my mother has died and she owed a month of fasting; shall I perform it on her behalf?" He said: "Do you see, if your mother had a debt, would you not pay it off for her?" She said: "Yes." He said: "Then the debt of God, the Almighty and Majestic, is more worthy of being paid."