Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a woman came to the Noble Prophet (peace and blessings be upon him) and mentioned her sister, who had vowed to fast for a month, but she passed away during a sea journey and was unable to fast. The Messenger of Allah (peace and blessings be upon him) said: "You should fast on behalf of your sister."