Abu Zubayr reported that he heard Abdul-Rahman bin Ayman, the freed slave of Urwa, asking Ibn Umar while Abu Zubayr was listening.Abdul-Rahman asked, "What is your view regarding a man who divorces his wife while she is menstruating?"Ibn Umar replied, "Abdullah bin Umar divorced his wife while she was menstruating during the time of the Messenger of Allah ﷺ. So Umar asked the Messenger of Allah ﷺ about it, saying, 'Abdullah bin Umar has divorced his wife while she was menstruating.'"Abdullah said, "He returned her to me and did not count it as anything. He said, 'When she becomes pure, then let him divorce her or keep her.'"Ibn Umar said that the Prophet ﷺ recited, "Prophet, when you divorce women, divorce them" [Talaq 65:1] at the start of their waiting period.
حدثنا أحمد بن صالح حدثنا عبد الرزاق أخبرنا ابن جريج
أنه سمع عبد الرحمن بن أيمن مولى عروة يسأل ابن عمر وأبو الزبير يسمع قال كيف ترى في رجل طلق امرأته حائضا قال طلق عبد الله بن عمر امرأته وهي حائض على عهد رسول الله ﷺ فسأل عمر رسول الله ﷺ فقال إن عبد الله بن عمر طلق امرأته وهي حائض قال عبد الله فردها علي ولم يرها شيئا وقال إذا طهرت فليطلق أو ليمسك قال ابن عمر وقرأ النبي ﷺ {يا أيها النبي إذا طلقتم النساء فطلقوهن} في قبل عدتهن
Muslim narrated it from the hadith of Abdul-Razzaq through this same route. See no. 2183.Regarding his statement, "and he did not consider it anything," this means he did not consider it something valid, because it was not performed according to the sunnah. This was stated by Ibn Abdul-Barr (Fath al-Bari 9/354).