Abu Dawud Book 15, Hadith Number 3302.

Narated By 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 Apostle of Allah (PBUH). So he commanded to fast on her behalf.