Ayatollah Khamenei:
By accepting to be heir to (Ma'mun's) throne, Imam Reza (pbuh) made a move that was unprecedented in the history of Imam's since the end of the Ahl al-Bayt caliphate in 40 a.h. and through the end of the caliphs rule. The move was about revealing the Shia claim of imamate - which was hidden under the cover of religious reservation - and spreading the message of Shi'ism to all Muslims.
August 9, 1984