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.  

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  

The Prophet ﷺ as saying: If anyone dies when some fast due from him has been unfulfilled, his heir must fast on his behalf.  

أَنَّ النَّبِيَّ ﷺ قَالَ مَنْ مَاتَ وَعَلَيْهِ صِيَامٌ صَامَ عَنْهُ وَلِيُّهُ