Maliks Muwatta Book 22, Hadith Number 1.

Yahya related to me from Malik from Ibn Shihab from Ubaydullah ibn Abdullah ibn Utba ibn Masud from Abdullah ibn Abbas that Sad ibn Ubada questioned the Messenger of Allah, may Allah bless him and grant him peace, and said, “My mother died while she still had a vow which she had not fulfilled.” The Messenger of Allah, may Allah bless him and grant him peace, said, “Fulfil it for her.”