Sahih Al Bukhari Hadith Number 1952

Narrated `Aisha: Allah’s Messenger (peace be upon him) said, “Whoever died and he ought to have fasted (the missed days of Ramadan) then his guardians must fast on his behalf.”

30  Fasting
Hadith number 1952
Volume no 3
Chapter: 42 (Can somebody else observe Saum instead of the dead?)