Narrated Abdullah ibn Abbas: A woman made a voyage and vowed that she would fast one month if Allah made her reach her destination with peace and security. Allah made her reach her destination with security but she died before she could fast. Her daughter or sister (the narrator doubted) came to the Messenger of Allah (peace be upon him). So he commanded to fast on her behalf.
Sunan Abi Dawud, Book 22, Hadith 67