The Prophet ﷺ as saying: If anyone dies when some fast is due from him (i.e. which he could not keep) his heir must fast on his behalf. Abu Dawud said: This applies to the fast which a man vows ; and this is the opinion of Ahmad b. Hanbal. (Using translation from Abū Dāʾūd 2400)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «مَنْ مَاتَ وَعَلَيْهِ صِيَامٌ صَامَ عَنْهُ وَلِيُّهُ»