'A'isha (RAA) narrated that The Messenger of Allah (ﷺ) said:
"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. (IT)
English reference (IT): Book 5, Hadith 699 (IT)
Arabic reference (IT): Book 5, Hadith 679 (IT)