Musannaf Ibn Abi Shayba 35:72

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from ‘Ubayd Allah, from Ibn ‘Abbas, that Sa‘d ibn ‘Ubadah asked the Prophet, may Allah bless him and grant him peace, for a legal opinion regarding a vow that was upon his mother, and she died before she could fulfill it. He said: "Fulfill it on her behalf."