As for the third hadith, reported by ʿĀʾishah (may Allah be pleased with her), the Prophet ﷺ said: “Whoever dies while a fast is still due from him, his wali (guardian or heir) should fast on his behalf.” In other words, if a person passes away owing any days of fasting, his wali may perform those fasts for him, whether the fast was a vow or an obligation established by the Sacred Law itself.
For example, suppose a man broke his fast in Ramadan because he was travelling, then after Ramadan became careless and did not make up the missed days—even though the law permits delaying the make-up until Shaʿbān—and he died before completing them. In that case, his wali—meaning any heir such as his mother, father, son, daughter, or wife—may fast those days for him. This is encouraged rather than strictly obligatory. If the wali chooses not to fast, he should instead feed one needy person for each missed day.
The same principle applies if the deceased had an expiation to perform and died before fulfilling it, despite having had the ability to do so; his wali may fast for him. Likewise, if he had vowed to fast three days and died before doing so, his wali should fast them; if the wali will not, he must feed a needy person for each of those days.
The fourth hadith, also narrated by ʿĀʾishah (may Allah be pleased with her), relates that the Prophet ﷺ said: “Fever arises from the heat of Hell, so cool it with water.” Fever is the illness that raises a person’s body temperature. The Prophet ﷺ attributed it to the heat of Hell. How that heat reaches the human body is known only to Allah; we do not know. We simply affirm, as the Prophet ﷺ said: “Fever is from the heat of Hell, so cool it with water”—that is, pour water over the patient to lower his temperature. This is one of the means of healing for someone stricken with fever, and modern medicine substantiates it: among the standard treatments for fever is the physician’s advice that the patient bathe in water, the cooler the better so long as it causes no harm, whereby the fever diminishes.