SHAFAQNA – It is narrated that, Imam Al-Hassan Al-Askari (AS) told Mosa ibn Jaffar Al-Baghdadi: It seems that I see you fall in dispute about my successor. But be aware that, whoever believes in Imams after the Prophet of Islam (PBUH), but denies Imamate of my son, is like the one who accepts the Prophethood of all the Prophets, but denies the Prophethood of Prophet Mohammad (PBUH). But denying the Prophet Mohammad (PBUH) is like denying all the Prophets. Obeying our last Imam (Imam Mahdi (AJ)), is like obeying our first Imam (Imam Ali (AS)); whoever denies our last one, is like denying our first Imam (AS). Be aware that the occultation of my son is so long that people will be doubtful except those who Allah (SWT) protects their beliefs .
 Ithbatul Hoda, Vol. 6, Page 427 & Beharul Anwaar, Vol. 51, Page 160.