A woman came to the Messenger of Allah ﷺ and said, "My mother passed away owing a vow of fasting, and she died before fulfilling it."The Messenger of Allah ﷺ said, "Let her guardian fast on her behalf."
حدثنا محمد بن يحيى حدثنا يحيى بن بكير حدثنا ابن لهيعة عن عمرو بن دينار
أن امرأة أتت رسول الله ﷺ فقالت إن أمي توفيت وعليها نذر صيام فتوفيت قبل أن تقضيه فقال رسول الله ﷺ ليصم عنها الولي
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[Its chain is daif]. Busiri graded it daif. See hadith 330 perhaps, and the hadith "Whoever dies while owing fasts, his guardian should fast on his behalf" makes it unnecessary.