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