Mishkat al-Masabih 7 Fasting كتاب الصوم 76

وَعَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ مَاتَ وَعَلَيْهِ صَوْمٌ صَامَ عَنْهُ وليه»

‘A’isha reported God’s messenger as saying, “If anyone dies when some fast due from him has been unfulfilled, his heir must fast on his behalf.” (Bukhari and Muslim.)