“My mother died while still owing fasts she had vowed. May I fast on her behalf?” He answered, “Yes.” In this context, the “guardian” is the heir.

Ibn al-Qayyim (may Allah have mercy on him) said: “One may fast on behalf of the deceased for vowed fasts, not for the original obligatory fast. This is the position of Ahmad and others, and it is the explicit view reported from Ibn ʿAbbās and ʿĀ’ishah. It is dictated by both evidence and analogy: a vow is not required by the Sacred Law to begin with; the servant voluntarily made it binding upon himself, so it takes the status of a debt. For that reason the Prophet ﷺ likened it to a debt. By contrast, the fast Allah originally obligated is one of the pillars of Islam; substitution can never enter into it—just as no one can deputise another to perform the prayer or to utter the two testimonies of faith. The purpose of these acts is the servant’s own obedience and his personal fulfilment of the worship for which he was created and commanded; no one else can discharge that for him, nor can anyone pray in his stead.”

Shaykh al-Islām Ibn Taymiyyah (may Allah have mercy on him) said: “A needy person should be fed for each day on his behalf. This is the view adopted by Ahmad, Isḥāq, and others, and it is what reason and transmitted texts alike require. The vow remained a liability upon the deceased, so it is fulfilled after death. As for the fast of Ramadan, Allah did not obligate it upon one who is incapable; rather, He instructed the incapable to pay the fidyah of feeding a poor person. Making up the fast is required only of one who had been able to observe it, not of one who was unable; hence no one needs to make it up on behalf of another. As for fasting a vow and other vowed acts, they are carried out on his behalf without dispute, owing to the sound hadiths on the matter.”