It is narrated from Mohammad ibn Abbas who said:

We were talking about the miracles of Imam Hassan Askari (AS) when a Nasibi person who was against Imam (AS) said: If he can answer my letter which I will write without ink, it will become clear that he is Imam, and he is right. We wrote a letter and asked Imam Askari (AS) some questions. That Nasibi man also wrote a letter with some questions without any sign of ink, and placed it inside an envelope and put it among our letters. We sent the letters to Imam Askari (AS). Imam (AS) answered our questions and in a letter wrote the names of the father and the mother of that Nasibi person as that was his question. AS soon as that Nasibi man saw Imam’s answer to his question, he was amazed and fainted. When he came round, he believed in Imamate and rightfulness of Imam Hassan Askari (AS) [1].

[1] Beharul Anwaar, Vol. 50, Page 288.

Source: en.shafaqna.com