Abu Dawud Book 6, Hadith Number 2180.

Narated By Abdullah ibn Umar : AbdurRahman ibn Ayman, the client of Urwah, asked Ibn Umar and AbuzZubayr was was listening: What do you think if a man divorces his wife while she is menstruating? He said: Abdullah ibn Umar divorced his wife while she was menstruating during the time of the Apostle of Allah (PBUH).So Umar asked the Apostle of Allah (PBUH) saying: Abdullah ibn Umar divorced his wife while she was menstruating. Abdullah said: He returned her to me and did not count it (the pronouncement) anything. He said: When she is purified, he may divorce her or keep her with him. Ibn Umar said: The Prophet (PBUH) recited the Qur’anic verse: O Prophet, when you divorce women, divorce them in the beginning of their waiting period.”