that is, at a point when their waiting period is definite: when it is known whether their waiting period is due to pregnancy or menstruation, that it begins from the moment of divorce, and that it is determined either by pregnancy or by menstrual cycles.
Thus, if he divorces her while she is menstruating, he has not divorced her at a point when her waiting period begins, because the menstrual period in which the divorce occurs is not counted. Her waiting period therefore does not begin with the divorce in this case, so he has not divorced her at the beginning of her waiting period.
Likewise, if he divorces her during a period of purity in which he had intercourse with her, and her pregnancy has not become evident, she does not know whether her waiting period is to be determined by menstrual cycles or by pregnancy. She remains uncertain; thus, he has not divorced her at a point when her waiting period is definite. For if he divorces her during a period of purity in which he had intercourse with her, then, if she is pregnant, her waiting period lasts until she gives birth; but if she is not pregnant, her waiting period is three menstrual cycles. Yet does she know which it is or not? The answer is that if he had intercourse with her after menstruation, she does not know, since she may have conceived through that intercourse. This differs from the case in which he divorces her during a period of purity in which he did not have intercourse with her: then she knows with certainty that her waiting period is determined by menstrual cycles, not pregnancy. And if we know with certainty that her waiting period is determined by menstrual cycles, then he has divorced her at the beginning of her waiting period.
Thus, the indication that divorce during menstruation, or during a period of purity in which he had intercourse with her, is prohibited lies in the command in His saying: “Divorce them at the beginning of their waiting period” [al-Talaq 65:1]. A command denotes obligation, especially since He follows it with His saying: “Count the waiting period accurately, and fear Allah, your Lord” [al-Talaq 65:1]. And He says: “These are the limits set by Allah; whoever transgresses the limits set by Allah has certainly wronged his own soul” [al-Talaq 65:1]. All of this confirms that the command denotes obligation.
The evidence from the sunnah is that Ibn Umar, may Allah be pleased with him and his father, divorced his wife while she was menstruating. When news of this reached the Prophet, peace and blessings be upon him, he became angry—that is, he was angered by what Ibn Umar, may Allah be pleased with him and his father, had done—and he said to Umar: