كتاب المنثورات والملح
19
The Book of Miscellaneous ahadith of Significant Values
'Aishah (May Allah be pleased with her) said:
The Prophet (ﷺ) said, "If a person dies without observing Saum (fasts), his Wali should make it up on his behalf."

[Al-Bukhari and Muslim].

وعنها رضي الله عنها عن النبي صلى الله عليه وسلم قال‏:‏ ‏ "‏من مات وعليه صوم، صام عنه وليه‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏
Sunnah.com reference : Book 19, Hadith 51
Arabic/English book reference : Book 19, Hadith 1858