1107 – Ibn ʿAbbās (may Allah be pleased with them both) narrated: “Saʿd ibn ʿUbādah sought a ruling from the Messenger of Allah ﷺ concerning a vow that had been incumbent upon his mother; she died before fulfilling it. He ﷺ said, ‘Fulfil it on her behalf.’” Agreed upon. (1)

Points derived from the ḥadith:
1 – Fulfilling a vow is an act of worship that must be carried out. Allah, Exalted is He, praises those who keep their vows in several noble verses.
2 – If someone dies while still owing a vow of obedience, it is prescribed for the heir to discharge it on his or her behalf.
3 – As for the vow owed by Saʿd’s mother, some scholars said it was fasting; others said manumission, charity, or a general vow. Each view cites supporting reports. Qāḍī ʿIyāḍ held that her vow concerned property belonging to her son, whereas Ibn Ḥajar considered it more likely that it pertained to a specific item designated by Saʿd himself.
4 – The ḥadith establishes the legitimacy of dutifulness to one’s parents, and that one of the greatest forms of filial piety is to settle whatever debts, rights, and obligatory duties remain upon them, whether those obligations are owed to Allah or to other people.
5 – This narration is among the proofs that the deceased benefits from righteous deeds performed on their behalf—such as manumission, charity, fasting, and the like. This is clarified in His words: “and that man will have nothing except what he strives for” (al-Najm 53:39). Ibn al-Qayyim explained that the Quran does not deny a person’s benefiting from the effort of another; rather, it negates his ownership of what he did not personally strive for. Someone else’s effort belongs to that person, and he may gift it to whomever he wishes or retain it for himself. Allah, Exalted is He, did not