Musnad Ahmad 27:1075

Rawh narrated to us, Ibn Jurayj narrated to us, Abu al-Zubayr informed me that he heard Abd al-Rahman ibn Ayman asking Ibn Umar, while Abu al-Zubayr was listening, and he said: "How do you view a man who divorces his wife while she is menstruating?" He said: "Ibn Umar divorced his wife during the time of the Messenger of Allah, peace and blessings of Allah be upon him. Umar said: 'O Messenger of Allah, Abdullah has divorced his wife while she is menstruating.' The Prophet, peace and blessings of Allah be upon him, said: 'Let him take her back.' He did not consider it anything, and he said: 'Return her. When she is pure, then let him divorce or keep her.'" Ibn Umar said: "And the Prophet, peace and blessings of Allah be upon him, recited: {O Prophet, when you [people] divorce women, divorce them for [the commencement of] their waiting period} [At-Talaq: 1]." Ibn Jurayj said: "And I heard Mujahid reciting it in that manner."