Sunan Ibn Majah
2133
Sahih+3

It was narrated from Jabir bin 'Abdullsh that :a woman came to the Messenger of Allah (ﷺ) and said: "My mother has died, and she had made a vow to fast, but she died before she could fulfill it. The Messenger of Allah (ﷺ) said: 'Let her guardian fast on her behalf

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi