If somebody dies without fulfiling a vow

Narrated Sa`id bin 'Ubada Al-Ansari:

that he consulted the Prophet (ﷺ) about a vow that had been made by his mother who died without fulfilling it. The Prophet (ﷺ) gave his verdict that he should fulfill it on her behalf. The verdict became Sunna (i.e. the Prophet's tradition).

حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، أَخْبَرَهُ أَنَّ سَعْدَ بْنَ عُبَادَةَ الأَنْصَارِيَّ اسْتَفْتَى النَّبِيَّ صلى الله عليه وسلم فِي نَذْرٍ كَانَ عَلَى أُمِّهِ، فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ‏.‏ فَأَفْتَاهُ أَنْ يَقْضِيَهُ عَنْهَا، فَكَانَتْ سُنَّةً بَعْدُ‏.‏

Narrated Ibn `Abbas:

A man came to the Prophet (ﷺ) and said to him, "My sister vowed to perform the Hajj, but she died (before fulfilling it)." The Prophet (ﷺ) said, "Would you not have paid her debts if she had any?" The man said, "Yes." The Prophet (ﷺ) said, "So pay Allah's Rights, as He is more entitled to receive His rights."

حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي بِشْرٍ، قَالَ سَمِعْتُ سَعِيدَ بْنَ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ أَتَى رَجُلٌ النَّبِيَّ صلى الله عليه وسلم فَقَالَ لَهُ إِنَّ أُخْتِي نَذَرَتْ أَنْ تَحُجَّ وَإِنَّهَا مَاتَتْ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ لَوْ كَانَ عَلَيْهَا دَيْنٌ أَكُنْتَ قَاضِيَهُ ‏"‏‏.‏ قَالَ نَعَمْ‏.‏ قَالَ ‏"‏ فَاقْضِ اللَّهَ، فَهْوَ أَحَقُّ بِالْقَضَاءِ ‏"‏‏.‏