Sa'd bin 'Ubadah asked the Messenger of Allah ﷺ about a vow which his mother had made, but she had died without fulfilling it. The Messenger of Allah ﷺ said: "Fulfill it on her behalf ."  

أَنَّ سَعْدَ بْنَ عُبَادَةَ اسْتَفْتَى رَسُولَ اللَّهِ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ تُوُفِّيَتْ وَلَمْ تَقْضِهِ فَقَالَ رَسُولُ اللَّهِ ﷺ اقْضِهِ عَنْهَا  

a woman came to the Messenger of Allah ﷺ and said: "My mother has died, and she had made a vow to fast, but she died before she could fulfill it. The Messenger of Allah ﷺ said: 'Let her guardian fast on her behalf ."  

أَنَّ امْرَأَةً أَتَتْ رَسُولَ اللَّهِ ﷺ فَقَالَتْ إِنَّ أُمِّي تُوُفِّيَتْ وَعَلَيْهَا نَذْرُ صِيَامٍ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَقَالَ رَسُولُ اللَّهِ ﷺ لِيَصُمْ عَنْهَا الْوَلِيُّ