Hadith: ‘Abdullāh ibn ‘Umar (may Allah be pleased with him) divorced his wife while she was menstruating. So, ‘Umar (may Allah be pleased with him) informed the Messenger of Allah (may Allah’s peace and blessings be upon him) about that, and he became furious at him

عن عبد الله بن عمر رضي الله عنهما : «أنه طلق امرأته وهي حائض، فذكر ذلك عمر لرسول الله صلى الله عليه وسلم ، فَتَغَيَّظَ منه رسول الله صلى الله عليه وسلم ، ثم قال: لِيُرَاجِعْهَا، ثم لِيُمْسِكْهَا حتى تَطْهُرَ، ثم تَحِيضُ فَتَطْهُرَ، فإن بدا له أن يطلقها فليطلقها طاهرًا قبل أن يَمَسَّهَا، فتلك العِدَّةُ، كما أمر الله عز وجل ». وفي لفظ: «حتى تَحِيضَ حَيْضَةً مُسْتَقْبَلَةً، سِوَى حَيْضَتِهَا التي طَلَّقَهَا فيها». وفي لفظ «فحُسِبَتْ من طلاقها، ورَاجَعْهَا عبدُ الله كما أمره رسول الله صلى الله عليه وسلم ».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported that he divorced his wife while she was menstruating. So, ‘Umar (may Allah be pleased with him) informed the Messenger of Allah (may Allah’s peace and blessings be upon him) about that, and he became furious at him and said: “Let him take her back and keep her until she becomes purified of menstruation, and then menstruates and then becomes purified of menstruation. Thereafter, if he decides to divorce her, he should divorce her while in a state of purity and before having intercourse with her. That is the ‘Iddah (post-divorce waiting period), as commanded by Allah, the Almighty. In another narration: “Until she enters the next menstrual period, other than the one during which he divorced her.” In yet another narration: “It was counted as one valid divorce, and ‘Abdullāh took her back at the command of the Messenger of Allah (may Allah’s peace and blessings be upon him).”
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) divorced his wife while having her menses. Thereupon, his father told the Prophet (may Allah's peace and blessings be upon him) about that, and he became extremely angry because he had divorced her in a manner that is unlawful and inconsistent with the Sunnah. Then, he commanded him to take her back and keep her until she would become purified after that menstrual period and would get another menses and then become purified of it. Afterwards, if he had no desire to keep her and decided to divorce her, he should do so before approaching her sexually. That is the period of abstention during which divorce can take place, as commanded by the Almighty Lord. Scholars differed over whether divorce of a menstruating woman is effective, even though it is unlawful and not in conformity with the Sunnah. The adopted view in this regard is the one based on the narration reported by Abu Dawūd and others: “He returned her to me and did not consider it to be anything.” This narration does not explicitly mention that the divorce was effective or was counted as such by the Prophet (may Allah’s peace and blessings be upon him). It is noteworthy here to cite the well-known Hadīth that reads: “If anyone does something contrary to our matter, it will be rejected.” [Al-Bukhāri and Muslim]