Ruling on making a vow to do obligatory religious duties
16-1-2014 | IslamWeb
Question:
A Christian woman who is married to a Muslim man made a vow that if she gave birth to a boy, she would fast Ramadhaan five times (i.e. observe fasting on Ramadhaan for five years). Later on, she embraced Islam and wanted to fulfill her vow, but she cannot make up for all these fasts. What is the religious ruling in this regard? What should she do? What is the ruling if a person vows to do something that is already obligatory under the Sharee‘ah?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
A vow made by a disbeliever is valid and binding and it is incumbent upon him to fulfill it should he revert to Islam. ‘Umar ibn Al-Khattaab 
said to the Prophet, sallallaahu ‘alayhi wa sallam, “O Messenger of Allaah, I vowed to perform I‘tikaaf (i.e. seclusion in the mosque for the purpose of worship) for one night during the pre-Islamic period of ignorance (i.e. before embracing Islam).” In another narration, ‘Umar ibn Al-Khattaab said, “…I vowed to perform I‘tikaaf (i.e. seclusion in the mosque for the purpose of worshipping) for one night in Al-Masjid Al-Haraam, during the pre-Islamic period of ignorance (i.e. before embracing Islam).” The Prophet, sallallaahu ‘alayhi wa sallam, said: “Fulfill your vow.” [Al-Bukhari and Muslim]
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
When a person makes a vow to do an act that is already obligatory on him such as fasting or prayer, such a vow is invalid and is not considered a binding vow according to Muslim jurists. This is because a vow is committing oneself to do a particular thing and one cannot commit himself to what is already obligatory on him. These acts are mandatory by virtue of the Islamic Sharee‘ah and vowing to perform them is meaningless and no expiation is due then, according to the correct opinion of Muslim scholars. This is also the adopted opinion of the Shaafi‘i school of Fiqh as stated by the author of Al-Muhath-thab and the majority of the Shaafi‘i scholars. This is also the adopted opinion of the Hanbali scholars, and others.
Hence, this woman is not required to do anything in this regard.
Allaah Knows best.