Saad bin Ubada sought a ruling from the Messenger of Allah ﷺ, saying, "My mother has died, and she had a vow she did not fulfill."The Messenger of Allah ﷺ replied, "Fulfill it on her behalf."
حدثنا القعنبي قال قرأت على مالك عن ابن شهاب عن عبيد الله بن عبد الله
أن سعد بن عبادة استفتى رسول الله ﷺ فقال إن أمي ماتت وعليها نذر لم تقضه فقال رسول الله ﷺ اقضه عنها
Reference, Bukhari narrated it in the book of Wills, chapter "What Is Recommended for One Who Dies Suddenly, That Charity Be Given on His Behalf," etc., no. 2761, and Muslim in the book of Vows, chapter "The Command to Fulfill a Vow," no. 1638, both from the hadith of Malik through this chain, and it is found in the Muwatta (Yahya's recension), 2/472.