Abdullah bin Abbas reported,

Saad bin Ubada al-Ansari sought a ruling from the Prophet ﷺ regarding a vow his mother had taken which she had not fulfilled before her death. The Prophet ﷺ instructed him to fulfill it on her behalf, and this became the established practice thereafter.

Azami 7126

Sahih

عن ابن عباس

أن سعد بن عبادة الأنصاري استفتى النبي ﷺ في نذر كان على أمه، فتوفيت قبل أن تقضيه، فأفتاه أن يقضيه عنها، فكانت سنة بعد.

الأعظمي 7126صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Oaths and vows
Chapter
Vows
Section
Fulfilling a vow on behalf of the deceased
Numbering
Jami al-Kamil 7126 (2nd edition)
Azami 7126