|Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, thanks a lot for your question which reflects your care to have a clear view of the teachings of Islam. Allah commands Muslims to refer to people of knowledge to get themselves well-acquainted with the teachings of Islam as well as all aspects of life.
In Islam, we are not allowed to tarnish the honor of anyone. One is required to produce four witnesses when making an allegation of adultery against another person; otherwise, one will be guilty of slandering.
A raped woman is a victim that must be treated with honor and kindness. She is not required to produce four witnesses to prove the crime done against her, nor is she punished for the crime done against her.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
If a person makes an allegation of adultery against another person (male or female) he or she must produce four witnesses to support such an allegation; otherwise, he or she is guilty of slandering, which is a grave offense in Islam, for we are not to tarnish the honor of anyone.
A woman who has been raped cannot be asked to produce witnesses; her claim shall be accepted unless there are tangible grounds to prove otherwise. To insist that she provide witnesses is akin to inflicting further pain on her. If anyone refutes her claim of innocence, the onus is on him to provide evidence, and she may simply deny the claim by making a solemn oath, thus clearing herself in public. The Prophet (peace and blessings be upon him) said, “The onus to provide evidence falls on the one who makes a claim, and the one who denies (the same) can absolve himself or herself by making a solemn oath to the contrary.”
As for a spouse who witnesses his or her partner committing adultery and the other party denies it and they are unable to provide witnesses, they are, if they so desire, to part company by repudiating each other by engaging in what is known as a solemn oath and prayer of curse (li`an). It is described thus in the Qur’an: “And those who accuse their wives, and have no witnesses but themselves, then the testimony of each of them shall be a testimony sworn by God repeated four times, that he is indeed truthful. And the fifth (oath) is that God’s curse be upon him if he is lying. And it shall avert punishment from her that she testify a testimony repeated and sworn by God four times, that he is lying. And a fifth (oath) that the wrath of God be upon her, if he has spoken the truth” (An-Nur: 6-9).
Excerpted, with slight modifications, from: www.muslims.ca
You can also read:
Are raped women punished in Islam?
Public Imprecation (Al-Li`an)