It is mentioned by various historians, Both of Shia and Sunni schools of thought, that when Mamun initially offered the Imam his throne, the Imam declined the offer firmly and resolutely. After several attempts to bring the Imam to accept the offer failed, Mamun told him to accept the offer of being his heir- apparent to succeed him after Mamun’s death.
Imam replied to this offer, “I will accept this to console you, but this will never happen for I will leave this world before you.” Once Mamun achieved his desire to get the consent of the Imam as his successor, he declared this openly to all in order to see the reaction of the Abbasid clan who were favoring his half-brother Amin.
It was on the 27th of Ramadan, in the year 201 Hijri that the Imam was officially designated as Mamun’s “heir- apparent,” and the Imam’s name was included with his own on gold and silver coins. The inscription on these coins is well worthy of notice: “The King of God and the faith, Al Mamun, Amir and Khalifa of the faithful, and Al Ridha’, the Imam of the Muslims.”
This meant more than that the Imam was to be official chaplain to him for Mamun summoned the descendants of Abbas, men and women to come to Merv. It was an assembly of thirty three thousand and when they all assembled, the caliph Mamun called for Ali Al Ridha’ (AS) and gave him a place of honour among the greatest of the Abbasid nobles.
He then announced to those he had summoned, that he had carefully considered all the descendants of Abbas and also the descendants of ‘Ali, and that he had not found in his search anyone more worthy or more fit to be his successor than Ali Al- Ridha.
He therefore took him by the hand and publicly acknowledged him as his successor(Kulaini). He then gave his daughter Umme Habib in marriage. He also sent abroad the command that the wearing of black flags which was the symbol of Abbasid rule after the destruction of the Ummayads, should be discontinued and that hereafter the use of green should be substituted. Green being the color of Ahlul Bayt and the House of ‘Ali (as) was the order of the day.
Once this story of succession was announced Mamun’s political ambition was achieved. He secured the help of the Persians fully behind him. With their help his army invaded the capital of the Abbasid empire, Baghdad, Amin was killed and his head was sent to Mamun to see for himself that he had become the sole ruler of the Abbasid Empire.
But Mamun was still not secure on his throne. The Arab party who sided with Amin, never liked the appointment of Imam ‘Ali al- Ridha’ (as) as the successor of Mamun, no matter how dubious it looked to them on the face of it.
The chiefs of the Abbasid family in Iraq perceived that by this appointment the principal authority in the empire would very likely be taken from them. They got together, therefore, and proclaimed that for bequeathing the Caliphate after his death to the Imam ‘Ali Al-Ridha’ (as), who was not their immediate family, Mamun himself was declared deposed.
They swore allegiance to Ibrahim al Mehdi, Mamun’s uncle as their new caliph. This proclamation took place on the 5th of Muharram 202 Hijri.
While Imam Ridha’ was with Mamun in Merv, his Prime Minister Fadl ibn Sahl arranged a conference on religions to which he invited the leaders of different sects, including Zoroastrians and Christians and Jews, that they might hear what the Imam of the Ahlul Bayt had to say to impress them of their spiritual abilities and excellences.
It was in these discussions with leaders of other faiths that the Imam clarified the position of sinlessness of the Prophets and of the guided Imams. These conferences were so successful that at one stage Mamun was fearful of the increasing influences of the Imam on the people as a whole.
In one such incidence on the occasion of the Eidul Fitr, the whole strategy of Mamun and the shear hypocrisy of the drama he was playing was exposed. He had asked the Imam to lead the Eidul Fitr prayers at the end of Ramadan that year. First the Imam declined, but when Mamun insisted, he agreed on the condition that he will direct the occasion as he would deem fit.
On the morning of the Eidul Fitr, when the military and civil leaders assembled outside the door of the Imam to come out and lead the prayers, they saw the Imam come out of the house bare foot, wearing a white shirt and white headgear. Imam also advised his companions to follow him in the same manner. He came out on the Road to the mosque outside the city of Merv.
All the citizens who saw the Imam in this fashion copied him and all walked barefoot. Even the military chiefs and civil judges walked bare foot. A huge crowd followed the Imam. He was saying Takbir loudly and all were following him and shouting Takbir (God is great).
The situation became so tense that when Mamun was informed of this huge following of the Imam he immediately sent a request to the Imam to withdraw from leading the Eid prayers. Imam withdrew and it was a great show down for the Emperor and exposed his trickery to the full.
It was after about a year’s stay of the Imam in Merv that Mamun decided to do something about the situation in Iraq. He knew that his uncle Ibrahim al Mehdi had been proclaimed as Caliph there in his place. This was a thorn in his heart.
He decided that it was time for him to return from Khorasan and assert his rights in person. He had already strengthened his position due to the help of the Persians. With a huge army beside him Mamun prepared his journey back home.
He was accompanied, as Yakubi mentions it in his history, by Imam Ridha (AS) as his heir apparent and by his prime minister Fadl ibn Sahl, who was known as the holder of two offices, civil and military, being the Prime Minister as well as the chief of the Army.
But when they reached the town of Sarakhs, Fadl ibn Sahl was assassinated in his bath by two persons, Ghalib Al Rumi and Sarraj Al Khadim. They were found out by Mamun who immediately put them to death immediately that if there was any implication of Mamun in this murder it should not be exposed.
Mamun had his suspicions about his Prime Minister who was secretly gaining favor with the Abbasids in Baghdad. Historians did mention the name of Mamun in disposing off Fadhl ibn Sahl.
Within two days of this murder when the army reached the town of Tus, Imam Ridha fell ill and died within three days of his illness. His sickness was no more than three days and it was reported that Ibn Hisham, Mamun’s favorite henchman had given the Imam poison which caused his death.
He mixed poison in grapes and when Imam had eaten the grapes he became ill in the same manner as Imam Hasan did and died within three days.
Ibn Babawaih relates various reasons that have been assigned to Mamun for poisoning the Imam and shows also the circumstances in which Imam Ali Ridha (as) is said to have designated his son Muhanmmad ibn Ali as his successor to the Imamate.
Imam Ali Ridha (AS) died and was buried far off from Madina, the home of his forefathers of the Ahl al Bayt of the Prophet. In Sanabad, about a mile from the village where he died, they placed him in a grave inside the tomb of Harun Al Rashid who was buried there ten years ago.
Mamun’s ambitions to get the Empire under his feet was almost accomplished for he knew that the army under his command would not run away from him at this juncture. So he cleverly killed off the Imam and reached Baghdad with the declaration that the Caliphate of Banu Abbas would remain in the family. Ibrahim Al Mehdi was deposed and later killed and Mamun became the supreme leader.