(O Prophet! When ye (men) put away women, put them away for their (legal) period…) [65:1].
Qatadah reported that Anas said: “The Messenger of Allah, Allah bless him and give him peace, divorced Hafsah and so Allah, exalted is, revealed this verse. It was said to him: ‘Take her back, for she is one who fasts often and stands up in night vigil prayer’ often, and she is one of your wives in the Garden’ ”.
Al-Suddi said: “This verse was revealed about ‘Abd Allah ibn ‘Umar who divorced his wife while she was in her menses. The Messenger of Allah, Allah bless him and give him peace, ordered him to take her back and keep her until the period of her menses was over and then wait again until she was in her menses. When she became pure, he was free to divorce her provided that he did not sleep with her before that, for this was the waiting period which Allah has commanded to observe”.
Mansur ibn ‘Abd al-Wahhab ibn Ahmad al-Shalanji> Abu ‘Amr Muhammad ibn Ahmad al-Hiri> Muhammad ibn Zanjawayh> ‘Abd al-‘Aziz ibn Yahya> al-Layth ibn Sa‘d> Nafi‘> Ibn ‘Umar who divorced his wife one single divorce while she was still in her menses. The Messenger of Allah, Allah bless him and give him peace, commanded him to keep her until the period of her menses was over. He had then to wait until she was in her period, and if he was still determined to divorce, he had to wait until her period was over, provided that he did not sleep with her before divorcing her. This was the waiting period in which Allah, exalted is He, has ordered that women be divorced. This was narrated by Bukhari and Muslim from Qutaybah and al-Layth respectively.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou