Saad bin Ubada asked the Prophet ﷺ for a ruling regarding a vow his mother had made but had died before fulfilling it. The Prophet ﷺ said, "Fulfill it on her behalf."
قال الحارث بن مسكين قراءة عليه وأنا أسمع عن سفيان عن الزهري عن عبيد الله بن عبد الله
أن سعد بن عبادة استفتى النبي ﷺ في نذر كان على أمه فتوفيت قبل أن تقضيه فقال اقضه عنها
[Sahih] See the previous hadith. It is also in Al-Kubra, hadith no. 6487, and Muslim narrated it, hadith no. 1638, from the hadith of Sufyan bin Uyayna by this same chain.