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 ."
Sunan Ibn Majah Book 11, Hadith 43
← Previous Hadith her father met the Prophet (ﷺ) when she was riding behind him. He said: "I vowed to offer a sacrifice at Buwanah." The Messenger of Allah (ﷺ) said: "Is there any idol there?" He said: "No." He said: " Fulfill your vow." (Hasan)Another chain with similar wording.

Sunan Ibn Majah Book 11, Hadith 42 Next Hadith → 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 ."

Sunan Ibn Majah Book 11, Hadith 44