On many occasions Imam Musa Kazem (AS) explicitly introduced his eldest son Reza as his immediate successor to accede to the divine position of Imamat.
Subsequent to the martyrdom of his father in Baghdad, on the 25th of Rajab 183 AH in the prison of the Abbasid caliph Harun al-Rashid, he attained the holy position of Imamat, when he was thirty-five years old, and he held that divine position for twenty years.
The Umayyad and the Abbasid caliphs were afraid of the Holy Imams (AS), who were publicly recognized as the true and worthy successors of the Messenger of Allah. The Holy Imams (AS) were therefore constantly persecuted and tortured by the ruling caliphs of the time.
Ma'mun (Harun’s son) sought to find a new solution for these difficulties which his Abbasid predecessors had not been able to solve. Ma'mun contrived to invite Imam Reza (AS) to Merv, the city where Ma'mun's seat of caliphate was located, with the evil intention of making false friendship with His Holiness. Thus, the cruel caliph intended to have a direct eye on His Holiness. In order to have this decision put into effect, Ma'mun forcefully exiled Imam Reza (AS) from Medina to Merv.
According to Ma'mun's commands, the direction of the caravan of Imam Reza (AS) from Medina to Merv was predetermined. He arrived in Merv on the 10th of Shawwal in 201 AH via Basra, Khorramshahr, Ahwaz, Rey, Neyshabur, Sanabad and Sarakhs.
When His Holiness arrived in Neyshabur he narrated a Hadith before a crowd of about 120,000 people, which also included thousands of distinguished scholars and narrator of traditions.
Imam Reza (AS) confirmed that worshipping God will be counted as a perfect procedure when it is based on obedience to the Immaculate Holy Imams (AS).
His Holiness was welcomed at Merv by the caliph himself and some distinguished Abbasid nobles at a place a few leagues outside the metropolis. After some days, Ma'mun offered Imam Reza (AS), first the caliphate, but His Holiness seriously rejected Ma'mun's proposal. Then Ma'mun proposed that the Imam accept to be the successor to the caliphate, and again the Holy Imam (AS) turned down his proposal but was finally forced to accept the to be the successor to the caliphate. The Holy Imam (AS) put forward certain conditions, saying that he would not interfere in governmental affairs or in the appointment or dismissal of government agents.
Ma'mun often organized debates in his own royal palace and celebrated scholars of various religions were invited there to exchange and discuss their different views on religious issues, but Imam Reza (AS) was always victorious in all those debates and soon his opponents recognized the scholarly status of His Holiness.
Imam Reza (AS) directed Muslims to the right path and eliminated the deviant views of the antagonists who endeavored to distort Islam. In this way, the Holy Imam (AS) safeguarded and propagated the principles of Islam.
The vicious caliph was scared of the ever-increasing popularity of the Holy Imam (AS). Thus he decided to eliminate the Holy Imam (AS).
In 203 AH/818 CE, Ma'mun first imprisoned Imam Reza (AS) in Sarakhs. Then the Holy Imam (AS) was taken to Sanabad and confined in the palace of Humaid bin Qahtabah. During the morning of the last day of the month of Safar 203 AH/818 CE, the Holy Imam (AS) was poisoned by Ma'mun in that palace.
The Holy Imam (AS) was seized with intense pain and languor and in the evening of the same day the holy soul of the innocent Holy Imam (AS) departed to heaven. The Holy Imam (AS) was buried in the solitude of night at the same place where His Holiness had predicted during his first visit to Humaid bin Qahtabah's palace at Sanabad in 201 AH/816 CE.
Through the presence of the holy shrine of Imam Reza (AS), the once small village of Sanabad soon transformed into a great metropolis named Mashhad, which is also now one of the most extensively visited holy sites in the world.