It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that a woman set out on a sea journey. She made a vow that if Allah Almighty brought her back safely, she would fast for one month. Allah Almighty brought her back safe and sound, but she was not able to fast until her death. Her sister came to the Prophet (peace and blessings be upon him) and related the whole incident. The Prophet (peace and blessings be upon him) said: "You should fast on her behalf."