Musnad Ahmad 25:478

Mu'awiyah narrated to us, Za'idah narrated to us, from al-A'mash, from Muslim al-Batin, from Sa'id ibn Jubayr, from Ibn Abbas, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, my mother has died and she owed a month of fasting; should I perform it on her behalf?" He said: "If your mother had a debt, would you pay it off on her behalf?" He said: "Yes." He said: "Then the debt of Allah is more worthy to be paid off." Sulayman said: Al-Hakam and Salamah ibn Kuhayl said, while we were all sitting when Muslim narrated this hadith: We heard Mujahid mentioning this from Ibn Abbas.