That Abdullah bin Umar told him that he had divorced his wife while she was in her menses so 'Umar informed Allah's Apostle of that. Allah's Apostle became very angry at that and said, "(Ibn 'Umar must return her to his house and keep her as his wife till she becomes clean and then menstruates and becomes clean again, whereupon, if he wishes to divorce her, he may do so while she is still clean and before having any sexual relations with her, for that is the legally prescribed period for divorce as Allah has ordered."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 431
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 431