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]. (ID)
Reference (ID): Bulugh al-Maram 1391 (ID)
In-book reference (ID): Book 13, Hadith 18 (ID)
English translation (ID): Book 13, Hadith 1391 (ID)