Divorce during a period of purity in which he had intercourse with her is forbidden, and it is obligatory for him to take her back.

The Shaykh : The second case: If he divorces her during a period of purity in which he had intercourse with her, it is a divorce not during the waiting period, and it is forbidden because it is a disobedience to Allah. He must take her back; he must return her to his protection. Now, why is it a divorce not during the waiting period? Because if he has intercourse with her after menstruation, it is possible that she becomes pregnant. If she becomes pregnant, her waiting period is the delivery of the child. It is also possible that she is not pregnant, in which case her waiting period is three menstrual cycles. So he did not divorce her during a known waiting period, but during an unknown waiting period, either pregnancy or menstrual cycles. Therefore, divorce during a period of purity in which he had intercourse with her is forbidden, and it is obligatory for him to take her back to his protection.