It is narrated from Sa'id bin al-Musayyib, may Allah have mercy on him, that he said: "So-and-so was mistaken because the Messenger of Allah (ﷺ) married Maymunah while he was in a state of being lawful (not in ihram)."
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 873
Hadith Takhrijاسناده ضعيف لإرساله: اخرجه ابوداود، المناسك باب المحرم يتزوج ، رقم : 1845 ۔ والبيهقي: 212/7 ۔ وفي المعرفة السنن والآثار له (2890)۔