The Prohibition of Denying Paternity

It is not permissible for the husband to deny his paternity of any child born to his wife as long as they are married to each other. Such a denial would bring the ugliest shame imaginable upon both the wife and the child. He is, therefore, not allowed to take such a step on the basis of a mere suspicion, a sudden notion, or an evil rumor. If, however, on the basis of evidence which has come to his attention, he is convinced that his wife has betrayed him, the Shari'ah of Islam has no desire to force him to raise a child whom he believes not to be his own or to let the child be his heir, or—at the very least—to allow him to suffer from suspicion and doubt the rest of his life.
A way out of this dilemma, known in Islamic jurisprudence as li'an, is provided by the Shari'ah. If a man is convinced or strongly suspects, although without having proof, that his wife has had sexual relations with another man and is carrying his child, he can take the case to a Muslim judge (qadi). The qadi will ask the man and his wife to invoke the curse of Allah on one another in the manner prescribed in Surah al-Nur: As for those who accuse their wives but have no witnesses except themselves, the testimony of one of them shall consist of bearing witness by Allah four times that he is of the truthful, and a fifth (time) that the curse of Allah be upon him if he is one of those who lie. And it shall avert the punishment from her if she bear witness by Allah four times that he is indeed of those who lie, and a fifth (time) that the wrath of Allah be upon her if he is among the truthful. (24:6-9)
After this the two shall be separated permanently, and the child shall be identified by the name of his or her mother.