Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) states that Sayyiduna Sa'd bin Ubadah al-Ansari (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) about his mother, who had a vow due upon her and died before fulfilling it. He (peace and blessings be upon him) ordered him to "fulfil the vow", and this became a Sunnah for those who came after.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20147