The Messenger of Allah ﷺ said, "Whoever dies with a month's fasting still owed, let food be given on his behalf to a poor person for each day missed."
حدثنا محمد بن يحيى حدثنا قتيبة حدثنا عبثر عن أشعث عن محمد بن سيرين عن نافع
قال رسول الله ﷺ من مات وعليه صيام شهر فليطعم عنه مكان كل يوم مسكين
[Its chain is daif. Tirmidhi transmitted it in the Book of Fasting, chapter on what has been said about the expiation, hadith 718, from Qutayba with this wording, and he said, "We do not know it as a raised hadith except through this route, and the correct position is that it comes from Ibn Umar as a stopped report, his own saying." He said, "Ashath is Ibn Sawar." See hadith 259 for its defect.