Musnad Abu Ya'la al-Mawsili 110:56

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from 'Ubayd Allah, from Ibn 'Abbas, who said: Sa'd ibn 'Ubadah sought a legal ruling from the Messenger of Allah (may Allah bless him and grant him peace) regarding a vow that was upon his mother, and she died before she could fulfill it. He said: "Fulfill it on her behalf."