Fulfilling a vow to commit a sin is impermissible
6-8-2015 | IslamWeb
Question:
I heard a hadith from some people who do not know its chain of narrators or whether it is authentic or not. The hadith reads:
A woman went to the Prophet, sallallaahu ‘alayhi wa sallam, and told him that she made a vow to commit adultery if something happened. When this thing happened, she asked the Prophet, sallallaahu ‘alayhi wa sallam, what she should do. He, sallallaahu ‘alayhi wa sallam, told her to fulfill her vow by passing by a group of men and unveil a small part of her leg and thus her vow would be fulfilled as if she had committed adultery.
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 
is His Slave and Messenger.
What the questioner asks about is not a Hadeeth, and it has no origin. When the person makes a vow to commit a sin, it is incumbent on him to break it (it is forbidden to deliver on such a vow). The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to obey Allaah, the Exalted, must do so, and whoever vowed to disobey Allaah, the Exalted, should not disobey Him." [Al-Bukhari]
‘Imraan 
reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "A vow to do an act of disobedience to Allaah, the Exalted, must not be fulfilled." [Muslim]
The person who vows to commit a sin should not do anything but turn to Allaah, the Exalted, in repentance. This is the opinion of Ash-Shaafi‘i, Maalik, and the majority of Muslim scholars 
.
Imaam Abu Haneefah 
believed that the person should offer expiation like that for breaking an oath as well. He depended on the Hadeeth that reads, "A vow to do an act of disobedience to Allaah, the Exalted, must not be fulfilled, and its expiation is the same as that of breaking an oath." [An-Nasaa‘i] As for fulfilling such a vow by committing the very sin that he vowed to commit, none of the scholars have ever said that, and it cannot be suggested by any sane person.
Allaah Knows best.