Vows of Fasting on Behalf of Others

Question:
May Allah reward you. A questioner says: I have an elderly aunt who has vowed to fast the first ten days of Dhul Hijjah every year for my father, who has passed away, and the month of Muharram every year for my eldest brother. My question is: Does anyone fast on her behalf, or upon her death? How is this vow to be understood? Please advise us, and may Allah reward you.

Answer:
This vow is not clear. Is she fasting on their behalf, or is she fasting for herself? The question is not clear. If she is fasting for herself, it is obligatory for her. If she has vowed to fast the first ten days of Dhul Hijjah or Muharram, it is obligatory for her. Whoever vows to obey Allah, let him obey Him. As for fasting on behalf of others, it is not obligatory for her to fast on behalf of others. One does not fast on behalf of another. If her vow is to fast on behalf of so-and-so, on behalf of her father, or on behalf of her mother, it is not obligatory. Worship is based on divine prescription. This vow is not valid. Yes. However, if she vowed to fast the first ten days of Dhul Hijjah for herself—meaning the nine days, excluding the day of Eid—or to fast the month of Ramadan, or to fast Muharram—meaning the first ten days—or to fast thirty days of every month, or to fast Mondays and Thursdays, there is no harm. It is obligatory for her to fast, based on the saying of the Messenger of Allah (peace and blessings be upon him): “Whoever vows to obey Allah, let him obey Him.” As for fasting on behalf of her father, or on behalf of so-and-so, or so-and-so, no, that is not prescribed.