Aisha reported that the Messenger of Allah ﷺ said, "Whoever dies while owing fasts, his guardian should fast on his behalf."
أن رسول الله ﷺ قال: من مات وعليه صيام، صام عنه وليه.
Bukhari reported it in the Book of Fasting (1952), and Muslim in the Book of Fasting (1147), through the route of Amr bin Harith, from Ubaydullah bin Abu Jafar, that Muhammad bin Jafar bin Zubayr told him, from Urwa, from Ayesha, and he mentioned it.Abu Dawud (2400) also reported it through this same route, and said, "This concerns the vow, and it is the view of Ahmad bin Hanbal." End of quote.