Imam al-Shafi'i (may Allah have mercy on him) said: "That is, in the hadith of Sayyiduna Ibn Umar (may Allah be pleased with him) it is clear that divorce takes effect on a menstruating woman, because the command to take back is given only to one upon whom the divorce has become binding; as for one upon whom divorce has not become binding, he remains in the same state he was in before the divorce."
The Shaykh (may Allah have mercy on him) said: "We have already mentioned the hadith (narrated) by Salim, Nafi' and Abdullah bin Dinar (may Allah have mercy on them) from Sayyiduna Ibn Umar (may Allah be pleased with him)."
Yunus bin Jubayr (may Allah have mercy on him) said: I asked Sayyiduna Abdullah bin Umar (may Allah be pleased with him): "A man divorced his wife while she was menstruating (what is the ruling)?" He said: "Do you know Ibn Umar?" I said: "Yes." He said: Sayyiduna Abdullah bin Umar (may Allah be pleased with him) divorced his wife while she was menstruating, so Sayyiduna Umar (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) and asked him, and he (peace and blessings be upon him) commanded him to take her back and then to divorce her at the start of her waiting period. I said: "Will that divorce be counted?" He said: "Yes." He said: "What do you think, if he was helpless or acted foolishly?"
(b) Al-Bukhari recorded it from Hajjaj bin Minhal, except that he said: I said: "Is that divorce counted?" He said: "What do you think, if he was helpless and acted foolishly?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14918