Testimony of the Imam
Ultimately, Mamun decided to assassinate the Imam, as he realized that under no circumstances could he make the Imam his puppet. The stature of the Imam and societal respect for him were increasingly growing. Despite Mamun’s best efforts to tarnish the Imam’s societal reputation, the Imam’s character and reverence were only amplifying day by day.
Mamun knew that as time passed, the legitimacy of the Imam and Mamun’s deception would become more evident. On the other hand, the Abbasids and their supporters were displeased with Mamun’s decision to designate the Imam as his successor. In protest, in Baghdad, they even pledged allegiance to “Ibrahim bin Mahdi Abbasid”. Thus, from multiple perspectives, Mamun’s rule was in jeopardy. Therefore, he secretly endeavored to eliminate the Imam and poisoned him. This was done both to rid himself of the Imam and to appease the Abbasids and their supporters. After the Imam’s martyrdom, he wrote to the Abbasids:
“You criticized why I designated Ali bin Musa al-Rida as the heir apparent. Know that he has passed away. Now, obey me.” 
Mamun tried to keep the Imam’s followers uninformed about the Imam’s martyrdom. He attempted to hide his intentions with pretense and deception, making it seem as if the Imam died a natural death. However, the truth couldn’t be concealed, and the Imam’s close associates and relatives became aware of the matter.
Aba Salt Harwi, one of the Imam’s close companions, narrates the sequence of events between Mamun and the Imam and how the esteemed Imam was ultimately murdered. Ahmad bin Ali Ansari says he asked Aba Salt: “How could Mamun, despite showing respect and affection for the Imam and even naming him his successor, possibly have plotted his murder?”
“Aba Salt said, “Having observed the Imam’s grandeur and nobility, Mamun feigned respect and affection, designating him as his successor to create an impression among the people that the Imam cherished worldly pursuits. He hoped this would lead to the Imam’s decline in public esteem. However, when Mamun saw that the Imam’s asceticism and piety remained untarnished and that people saw nothing contrary to holiness in him, the Imam’s stature among the people grew even more. Mamun gathered scholars from various cities, hoping that one among them might outdo the Imam in a scholarly debate, leading to a perceived fall in the Imam’s academic standing among scholars. Then, through them, the Imam’s purported failings would become widely known among the general populace. However, none – whether from the Jews, Christians, Zoroastrians, Sabaeans, Brahmins, atheists, adherents of temporal philosophies, or even debaters from Muslim sects – could best the Imam. In every discussion, the Imam emerged victorious, compelling his opponents to acknowledge his superior arguments. Observing this, people commented, ‘By God, the Imam is more fitting and deserving of the caliphate than Mamun.’ Mamun’s informants relayed these sentiments to him, stoking his anger and igniting the flames of his envy. Moreover, the Imam, blessed be he, never hesitated to speak the truth in Mamun’s presence, often expressing views that Mamun found displeasing. This further intensified Mamun’s animosity and resentment towards the Imam. Ultimately, when his various schemes against the Imam bore no fruit, he secretly poisoned him.” 
Additionally, Aba Salt, who accompanied the Imam and participated in his burial, said, “On the return journey from Marv to Baghdad in Tus, Mamun poisoned and assassinated the Imam using tainted grapes.” 
The sacred body of the Imam was laid to rest next to the spot where Harun had previously been buried, in front of Harun’s grave. The martyrdom of Imam Reza, peace be upon him, occurred on the last day of the month of Safar in the year 203 Hijri. At that time, the Imam was fifty-five years old.
Blessings of God, His prophets, the pure, and the righteous be upon the noble soul of that great being.
Indeed, the silence and distortion of history have clouded the full scope of atrocities committed by certain tyrants, including Al-Ma’mun al-Abbasi, for future generations. Through deceit and cunning, Ma’mun not only poisoned and murdered Imam Ali (peace be upon him), but he also decimated many of the Imam’s associates, noble Alawites, and loyal Shia followers. He expelled them from their cities, forcing them to wander through plains and mountains. He constricted their existence to such an extent that many of these noble souls had to hide, each seeking refuge in secluded corners. Eventually, some tasted the drink of martyrdom, while others lived and died in anonymity. The life histories of many of these figures are unknown, with only scattered records preserved by Shia scholars.
1- Tabari, Vol. 11, p. 1030 – Al-Bidaya wa’l-Nihaya, Vol. 10, p. 249, among others cited from “Life of Imam Al-Ridha”, p. 349
2 – Uyun al-Akhbar, Vol. 2, p. 241
3 – Uyun al-Akhbar, Vol. 2, p. 245.”