Mishkat al-Masabih 15 Oaths and Vows كتاب الإيمان والنذور 27

وَعَنِ ابْنِ عَبَّاسٍ: أَنَّ سَعْدَ بن عبَادَة رَضِي الله عَنْهُم اسْتَفْتَى النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَأَفْتَاهُ أَنْ يَقْضِيَهُ عَنْهَا

Ibn ‘Abbas told that Sa’d b. ‘Ubada asked the Prophet for a decision about a vow taken by his mother who had died before fulfilling it, and that he gave decision that he should fulfil it on her behalf. (Bukhari and Muslim.)