It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him) sought a ruling from the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah! My mother has died, and a vow was due from her which she had not fulfilled. the Messenger of Allah (peace and blessings be upon him) said: "Fulfil it on her behalf."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5346