Chapter 30: The Book of Wills
Muhammad bin 'Abdullah bin Yazid said: "Sufyan narrated to us from Az-Zuhri, from 'Ubaidullah bin 'Abdullah, from Ibn 'Abbas, that Sa' d said: 'My mother died and there was an (outstanding) vow that she had to fulfill. I asked the Prophet and he told me to fulfill it on her behalf.'"
Sunan an-Nasa'i 3661