Bulugh al-Maram, Book 13, Hadith 1391 (English) ⓘ

Narrated Ibn Abbas (RA): Sa'd bin Ubada asked Allah's Messenger for a ruling regarding a vow taken by his mother who had died before fulfilling it, and he said, "Fulfill it on her behalf." [Agreed upon].

وَعَنْ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: { اِسْتَفْتَى سَعْدُ بْنُ عُبَادَةَ ‏- رضى الله عنه ‏- رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-فِي نَذْرٍ كَانَ عَلَى أُمِّهِ, تُوُفِّيَتْ قَبْلِ أَنْ تَقْضِيَهُ ? فَقَالَ: "اِقْضِهِ عَنْهَا" } مُتَّفَقٌ عَلَيْهِ 1‏.‏ ‏1 ‏-.‏ صحيح.‏ رواه البخاري ( 2761 )‏، ومسلم ( 1638 )‏.‏