SHAFAQNA – Mohammad ibn Abbas narrated: We were talking about miracles of Imam Hassan Askari (AS) and a non-Muslim man who was also against Imam Askari (AS) said: If he answers my letter which is written without ink then it will become obvious that he is Imam and his rightfulness is justified. We wrote a letter and asked some questions. That non-Muslim man also wrote things in a letter without using ink, no sign of writing could be seen on the paper, then he placed it inside an envelope and put it among our letters.
All letters were delivered to Imam Askari (AS) and Imam (AS) answered our questions as well as in a letter wrote the name of the non-Muslim man, and the names of his father and mother. When the non-Muslim man saw the reply by Imam Askari (AS), he was astonished and fainted. When he regained consciousness, he believed in Imamate of Imam Hassan Askari (AS) .
 Beharul Anwaar, Vol. 50, page 288.