“Whoever dies while owing fasting, his guardian should fast on his behalf.”

Narrated by al-Bukhārī (no. 1952) and Muslim (no. 1147), from the ḥadīth of ʿĀʾishah—may Allah be pleased with her.

Brief Explanation of the Hadith

Among the mercy of Allah, Exalted be He, toward His servants and the ease of His legislation is that He did not neglect the condition of the Muslim even after his death; rather, He legislated that which clears his liability and completes what he missed during his life. Among that is what is mentioned in his statement—may Allah’s peace and blessings be upon him: “Whoever dies while owing fasting, his guardian should fast on his behalf.”
That is, whoever dies while owing an obligatory fast—such as making up missed days of Ramadan, a vowed fast, or an expiation—and did not fulfill it, it is prescribed for his guardian to fast on his behalf, thereby clearing his liability.
The term “guardian” (walī) here refers to a close relative of the deceased—such as his father, son, brother, or other kin. Some have said it refers to the heir. The more correct view is that it refers generally to a close relative. Even if a non-relative fasts on his behalf, the deceased’s obligation is fulfilled, for the purpose is to complete the missed fast, which has thereby been achieved.
His statement, “his guardian should fast on his behalf,” is expressed in the form of a report (khabar), but among scholars it is understood as a directive—i.e., “let his guardian fast on his behalf.” However, this directive is to be understood as recommended (mustahabb), not obligatory, because fasting is an act of worship, and a person is not required to perform acts of devotion on behalf of another. If the guardian prefers, he may instead feed a needy person for each day, provided the deceased left behind wealth. The requirement for making up the fasts or feeding others applies only when the deceased had the opportunity and ability to make up the fast but failed to do so. However, if he was unable—such as due to prolonged illness until death—there is neither making up nor feeding, because he died without negligence.
Thus, the ḥadīth indicates the permissibility of making up missed fasts on behalf of the deceased, encourages the guardians to do so, and reflects the care of the Shariʿah in clearing the liability of the Muslim after death.