Hadith 2400
Narrated Aishah: 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 bin Hanbal.
Hadith 2401
Narrated Ibn Abbas: If a man falls ill during Ramadan and he dies, while he could not keep the fast, food will be provided (for the poor men) on his behalf ; there is no atonement (for his fasts) due from him. If there is some vow which he could not fulfill, his heir must atone on his behalf.