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 ." (Using translation from Ibn Mājah 2132)
أَنَّ سَعْدَ بْنَ عُبَادَةَ اسْتَفْتَى رَسُولَ اللهِ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَأَفْتَاهُ رَسُولُ اللهِ ﷺ «أَنْ يَقْضِيَهُ»