Narrated Ibn 'Abbas:
A woman came to the Prophet ﷺ and said (to him) that one month's fast was due from her mother who had died. May I fulfill them on her behalf? He asked: Suppose some debt was due from your mother, would you pay it ? She replied: Yes. He said: So the debt due to Allah is the one which most deserves to be paid.

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، قَالَ سَمِعْتُ الأَعْمَشَ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ الْعَلاَءِ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، - الْمَعْنَى - عَنْ مُسْلِمٍ الْبَطِينِ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ، : أَنَّ امْرَأَةً، جَاءَتْ إِلَى النَّبِيِّ ﷺ فَقَالَتْ : إِنَّهُ كَانَ عَلَى أُمِّهَا صَوْمُ شَهْرٍ أَفَأَقْضِيهِ عَنْهَا فَقَالَ : " لَوْ كَانَ عَلَى أُمِّكِ دَيْنٌ أَكُنْتِ قَاضِيَتَهُ " . قَالَتْ : نَعَمْ . قَالَ : " فَدَيْنُ اللَّهِ أَحَقُّ أَنْ يُقْضَى " .