Abu Yusuf al-Qatii

Abu Yusuf Ishaq al-Qati'i, a Christian, author of a book uncovering the doctrines of the Harranians, who in Ibn al-Nadim's time were known as the Sabians. ²

In it he relates that al-Ma'mun, in the last days of his life, passed through the land of Mudar on his way to the Byzantine lands for a raid. People met him, among them a group of Harranians in a distinctive dress. He asked them whether they were Christians, Jews or Magians, and they said no. He asked whether they had a book or a prophet, and they stammered. He gave them time until his return from the journey to either enter Islam or enter one of the religions mentioned in the Qur'an, or else he would kill them. ²

This is al-Qati'i's account as Ibn al-Nadim gives it, and it has not been compared with another source here.