Born in Medina in 148 AH, he was 35 years when the mantle of Imamate (divinely-decreed leadership) came to rest on his shoulders following the martyrdom of his infallible father, Imam Musa Kazem (AS) in Baghdad in the dungeon of the tyrant Haroun Rashid. In 200 AH, Imam Reza (AS), because of his popularity among the people as the Prophet's righteous successor, was forced to leave Medina and come to Khorasan in northeastern Iran by Mamoun, whose capital was Merv (presently in Turkmenistan).
On 24th of the Islamic month of Zil-Qa'dah in 201 AH, Imam Reza (AS), the 8th Infallible Successor of Prophet Mohammad (SAWA), started his fateful journey from his hometown Medina in Hejaz, to the Khorasani city of Marv in northeastern Iran – currently in the republic of Turkmenistan – under pressure from Mamoun, the self-styled caliph of the usurper Abbasid regime.
The intention of Mamoun was to kill two birds with one stone by forcing the Imam – although twenty years his senior – to become his heir apparent, so that the Prophet's heir would be isolated from the Ummah by being confined to the court, while the gullible masses would either praise Mamoun for bestowing such an honour on the Imam, or would become disenchanted with the Ahl al-Bayt by saying court life had tainted the purity of the Prophet's Household.
He, however, miserably failed and two years later poisoned his venerable guest to death, when the sanctity and popularity of the Imam increased manifold. Today the golden-domed mausoleum of Imam Reza (AS) in Mashhad attracts pilgrims from all over the world, while there is no trace of the graves of the Abbasids.
Here, the Imam turned down the caliph's deceitful offer of caliphate saying: "if the caliphate is really yours, you cannot give to others what God has entrusted you with, and if it is not yours, how can you give it others what does not belong to you at all." Mamoun then forced the Prophet's Heir to agree to become his Heir Apparent although the middle-aged Imam was over twenty years his senior. The plan was to confine the Imam to the palace in the hope of tainting his spotlessly pure character with the luxurious life of the court, and also to isolate him from and the ummah.
However, to the bewilderment of the regime, when the Imam's popularity increased, as a result of his exemplary behaviour with the ordinary people, as well as his dynamic debates with scholars of the various philosophical schools and leaders of various religions (Jews, Christians, Zoroastrians and Hindus), Mamoun martyred him in a treacherous manner through a fatal dose of poison, then feigned grief, claiming that it natural death.
The Imam was laid to rest in Sanabad in the suburbs of Tous, which soon grew into "Mashhad-ar-Reza" – Martyrdom Place of Reza (AS) – or simply Mashhad, which is a world famous centre of pilgrimage today, with millions of people coming from all over the globe to pay homage at the magnificent golden-domed shrine.