Abdullah bin Abbas (may Allah be pleased with them) narrates: A woman came to the service of the Prophet (peace be upon him) and said: My mother has passed away and she was obliged to observe a fast as a vow. The Prophet (peace be upon him) said to this woman: Would you pay off the debt of your mother if it was obligatory upon her? The woman replied: Yes. The Prophet (peace be upon him) said: Then you should also fast on behalf of your mother.