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)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / Q14918