From Sahih Muslim (Hadith 3451):
Ibn Abbas (Allah be pleased with them) reported that Allah's Apostle (ﷺ) married Maimuna in the state of Ihram. Ibn Numair made this addition:" I narrated it to Zuhri and he said: Yazid b. al-Asamm (Allah be pleased with him) told me that he (the Holy Prophet) married her when he was not a muhrim
Reference: Muslim 3451