Musnad Ahmad 25:167

Yahya narrated to us, I heard al-A’mash, he narrated to me from Muslim, from Sa’id ibn Jubayr, from Ibn Abbas, that a woman said: "O Messenger of Allah, my mother had a month of fasting upon her, and she died. Should I fast it on her behalf?" He said: "If your mother had a debt, would you have paid it off?" She said: "Yes." He said: "Then the debt of Allah, the Almighty and Majestic, is more worthy to be paid off."