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
Sa'id bin Jubayr narrates from Ibn Abbas (may Allah be pleased with him) that a woman set out on a sea voyage and vowed that if Allah saved her, she would fast for a month. Allah saved her, but she died before fasting. Her daughter or her sister came, and the Prophet (peace and blessings be upon him) "gave the order to fast" on her behalf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20148