The Prophet ﷺ said, "Whoever dies owing a month's fast, let a poor person be fed on his behalf for each day."
حدثنا قتيبة حدثنا عبثر بن القاسم عن أشعث عن محمد عن نافع
عن النبي ﷺ قال من مات وعليه صيام شهر فليطعم عنه مكان كل يوم مسكينا
Its isnad is daif.It was also related by Ibn Majah, Fasting, Chapter: One who dies while owing fasts, etc., hadith 1757, and Ibn Khuzayma, hadith 2056, from the hadith of Qutayba, through the same route.In the narration of Ibn Majah, it reads "Muhammad bin Sirin," but this is a mistake. The correct name is Muhammad bin Abu Layla, who is well known to be daif. Sharik bin Abdullah Qadi related it from him, through the same route, in Ibn Khuzayma, hadith 2057.