Saad bin Ubada asked the Messenger of Allah ﷺ for a ruling regarding a vow that his mother had taken upon herself but died before fulfilling. The Messenger of Allah ﷺ said, "Fulfill it on her behalf."
حدثنا محمد بن رمح أنبأنا الليث بن سعد عن ابن شهاب عن عبيد الله بن عبد الله
أن سعد بن عبادة استفتى رسول الله ﷺ في نذر كان على أمه توفيت ولم تقضه فقال رسول الله ﷺ اقضه عنها
Bukhari related it in the Book of Wills, in the chapter, "It Is Recommended for the Heirs of One Who Dies Suddenly to Give Charity on His Behalf, and Fulfilling Vows on Behalf of the Deceased," hadith 2761 and 6698, from the hadith of Ibn Shihab Zuhri, through this same chain, and Muslim related it in the Book of Vows, in the chapter, "The Command to Fulfill the Vow," hadith 1638.