Narrated Ibn Abbas (RA):
Sa'd bin Ubada asked Allah's Messenger (ﷺ) 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].
Reference: Bulugh al-Maram 1391
In-book reference: Book 13, Hadith 18
English translation: Book 13, Hadith 1391