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Allah's Messenger (ﷺ) said, "Whoever died and he ought to have fasted (the missed days of Ramadan) then his guardians must fast on his behalf."

If anyone dies in a state (that he had to complete) some fasts, his heir must fast on his behalf.

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. Hanba

The Prophet (ﷺ) as saying: If anyone dies when some fast due from him has been unfulfilled, his heir must fast on his behalf.

"Whoever dies while he still has some fasts to make up (of the days of Ramadan), then his heir (any of them) should fast on his behalf.” Agreed upon.

‘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.)