Question: I wanted to know how far is an adulterer or fornicator rightfully allowed to draw a veil across his misdeeds? If someone questions him whether he has committed adultery or fornication, for instance, is it permissible for him to lie to hide his sin? If he says that he chooses not to answer that question it is very likely that questioner will automatically understand that there is something wrong.
 
    Answer: It is desirable not to reveal such sins to others. The Holy Prophet (sws) would not willingly hear such voluntary confessions. If someone questions such a person he should try not to reveal the matter. However, if the matter has been brought before the court and he is questioned there, then he is not allowed to hide the truth.
    The Prophet (sws) is reported to have said: 

He among you who gets involved in such filth, should hide behind the veil stretched out for him by Allah, but if he unfolds the veil, we shall implement the law of Allah upon him. (Mu’atta: No. 1562)

    Similarly, he once told a person:

If you had hidden the crime of this [person], it would have been better for you. (Mu’atta: No. 1553)