Musnad Ahmad 25:1561

Ibn Numayr narrated to us, al-A‘mash narrated to us, from Muslim al-Baṭīn, from Sa‘īd ibn Jubayr, from Ibn ‘Abbās, from the Prophet, peace and blessings of Allah be upon him, who said: A woman came to him and said: "My mother has died, and she owed a month of fasting; shall I perform it on her behalf?" He said: "Do you see, if she had a debt, would you pay it off for her?" She said: "Yes." He said: "Then the debt of Allah, the Mighty and Majestic, is more worthy to be paid off."