Al-Qurtubee mentions a marvellous story in his Tafseer. He says: The Caliph al-Ma’moon used to hold occasional debates on various issues at his court where all scholars were invited. It so happened once that a Jew came to attend one of these debates. By his looks, dress, speech and manners, he seemed to be an elegant person. His speech during the debate was excellent and brilliant. When the meeting was over, al-Ma’moon called him and asked him if he was a Jew, which he confirmed. To test him, al-Ma’moon asked him to accept Islam, in which case he could hope to receive the best possible treatment from him. The Jew said that he was not willing to abandon the religion of his forefathers. The matter ended there, and the man went away.
Then after one year the same person returned, but this time as a Muslim. He made a remarkable contribution to the court meetings on the subject of Islamic fiqh. When the sessions were over, al-Ma’moon called the man and asked him whether he was the same man who had come the previous year. The man answered, “Yes.” Al-Ma’moon asked him, “You refused to accept Islam last time you came. Tell me what made you accept Islam.”
He answered, “When I left your court, I decided to do some research on the religions of our time. I am a scribe by profession. I write books and sell them. They bring me good price. This time, I decided to make a test. First I made three handwritten copies of the Torah in which I made some additions and deletions of my own. I took these manuscripts to the synagogue. The Jews found them interesting and bought them.
“Then, I did the same thing with the New Testament. I made three manuscripts, complete with my additions and deletions and took them to the church. The Christians were pleased with these manuscripts and bought them from me. After that I tried the same thing with the Quran. I made three superb handwritten copies of the Quran but with a few additions and deletions of my own. When I went out to sell them, I faced a problem; for when Muslim booksellers examined the manuscripts they hurled them across saying that the books had been corrupted. From this incident, I learnt that this Book is protected and preserved by Allah Himself. I, therefore, embraced Islam.”
(Muhammad al-Qurtubee, Al-Jaami’ Li Ahkaam al-Quran, Vol. 5, pp. 6-7)