Sahih Muslim
The Book of Vows (Kitab Al-Nadhr)
Book Number: 014 / Hadith Number: 4017
Ibn Abbas reported that Sa'd b. Ubida asked Allah's Messenger (may peace be upon him) for a decision about a vow taken by his mother who had died before fulfilling it. Allah's Messenger (may peace be upon him) said: Fulfil it on her behalf.