Sunan an-Nasa'i, Volume 4, Book 35, Hadith 3849 (DarusSalam) ⓘ

It was narrated that Ibn 'Abbas said: "Sa'd bin 'Ubadah asked the Messenger of Allah about a vow which his mother had sworn, but she died before she could fulfill it. The Messenger of Allah said: 'Fulfill it on her behalf.'"

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ اسْتَفْتَى سَعْدُ بْنُ عُبَادَةَ رَسُولَ اللَّهِ صلى الله عليه وسلم فِي نَذْرٍ كَانَ عَلَى أُمِّهِ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ اقْضِهِ عَنْهَا ‏" ‏ ‏.‏