Answer
The Mihna is the name of a period of persecution of Islamic scholars in the Abbasid Caliphate in the early 9th century. It began in 833, when Caliph al-Ma'mun ordered that all Muslims must accept the Mu'tazila school of theology, which held that the Qur'an was created by God, rather than being eternal. This was a controversial doctrine, and many scholars refused to accept it.