Sunan Ibn Majah
2132
Sahih+3

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

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi