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,