It was narrated from Ibn 'Abbas' that : Sa'd bin 'Ubadah asked the Messenger of Allah (ﷺ) about a vow which his mother had made, but she had died without fulfilling it. The Messenger of Allah (ﷺ) said: Fulfill it on her behalf .
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 .