During the time of al-'Imam al-Hasan al-'Askari (A)

"During the time of al-'Imam al-Hasan al-'Askari (A), the institution of the khilafah had decided to kill the successor of the Imam (A) by all the ways and means possible in an effort to put an end to the subject of Imamah and consequently to Shia faith. In addition to other aspects, al-Imam al-Hasan al-'Askari (A) was also under surveillance from this point of view. It was for this reason that the birth of al-'Imam al-Mahdi (A) was kept secret. Until he became six, till his honourable father was alive, al-'Imam al-Mahdi (A) was kept out of sight and no one could see him other than a few devoted and trustworthy Shias."

ALLAMA TABATABAI (R)