بسم الله الرحمن الرحیم
Ali Ibn Musa al-Reza (ca. December 29, 765 - August 23, 818) was the seventh descendant of the Islamic prophet Muhammad and the eighth of the Twelve Imams. His given name was ‘Alī ibn Mūsā ibn Ja‘far. Imam Reza’s father was martyred in 799, when Imam Reza was 35, and he was given the responsibility of the Imamate. However, his Imamate was rejected by the Waqifite Shia. Imam Reza was not looked upon favorably by Hārūn Rashīd, and the people of Medina were disallowed from visiting Imma Reza and learning from him. Harun attempted to kill him but was ####unsuccessful. According to Shi'a tradition, the first Imams of Islam were infallible and became the practical example for others during their lives. They taught the lesson of life to the people. They did not segregate and separate themselves from them and did not lead their lives on the style and pattern of tyrants and oppressors. They did not overlook or show heedlessness and disrespect to anyone. Sheikh Saduq narrates through the words of Ibrahim ibn Abbas that, "I never saw Imam Reza committing excess over anyone in talking and interrupting anybody's speech before its coming to the end. He did not stretch his legs in the presence of others. When the meal table was laid he invited the servants to it, and took his meals with them. After taking rest in the nights he got up and made himself busy with the prayers to Allah. Similar to his fore fathers he carried food to the houses of the afflicted ones at the mid of nights." He led a simple life in his home, but when he went out he used to decorate himself and put on new and clean dress. He respected and honored the guests to the extent that one night a lamp of the house went out of order, the guest got to his feet to put it right. Imam Reza made him sit and he set the lamp right, and said, "We do not employ our guests (exploit) upon work."
Imam Reza did not outlive Ma'mun, martyred on May 26, 818, in Persia while accompanying Ma'mun at Tus. Most scholars agree he was poisoned by al-Ma'mun who poisoned Imam using grapes. Ali al-Ridha is buried within Imam Ridha Mosque, in Mashhad, Iran. After the death of Hārūn Rashīd, Hārūn's two sons began fighting for control of the Abbāsid Empire. One son, Amin, had an Arab mother and thus had the support of Arabs, while his half-brother Al-Ma'mun had a Persian mother and the support of Persia. Ma'mun believed that Persia was sympathetic to the Hashemites and asked for Ali al-Rida to meet him in Persia. Ali al-Ridha left his only son, Muħammad at-Taqī, and his wife and set out for Merv. After defeating his brother, Ma'mun named Ali al-Ridha his successor. He hoped to win Shī'a support through this move, but the passage of caliphate would occur only if Ali al-Rida outlived Ma'mun (as with all promises of succession). Ma'mun even changed the black Abbāsid flags to green, the traditional color of the house of Alī ibn Abī-Tālib, the first Shī'a imam. On the night of his death Imam Reza saw his grandfather Muhammad in the dream saying, "Come to us tomorrow, what we have for you is better than the condition you are in." Harthama says, Imam Reza got up in the mid of the night and called me so that I be present before him. I put on my dress and went to Imam Reza. He said to me, "al-Ma'mun has decided to poison me through grapes tomorrow. Be aware, after my martyrdom he wishes to give me a bath. Tell him to refrain from it. If you do that Allah will not give you chance and respite (any-longer). At that time my son will arrive from Medina and give me a bath and coffin, without anyone seeing it or getting informed. Then, they will carry me to the tomb of Hārūn al-Rashīd. Ma'mun wants to bury me behind him. But howsoever they work on the earth the pickaxe; they will fail to dig the soil. At that time tell Ma'mun to dig the earth in front of the grave so that a ready-made grave will appear. At that instance a white water boils out of the head-side (grave) and rises up. Small fish appear in that. Then a big fish appears which eats up the small fish. Thereafter, the water will settle down. After that lay me in the grave. Do not put the soil on my grave since; it will be filled up by itself. Oh Harthama, do what I have said and do not allow things to happen against what I told you, otherwise, Allah will torment you (people)." At last Ma'mun poisoned Imam Reza through grapes in Tus, on the way back from Merv to Baghdad. Imam Reza came to the house and said to his old friend Aba Salt, "Collect the carpets of the house and do not allow anyone to enter the house, as this is the time when my soul departs (dying) I want to die on the earth like my grandfather Husayn ibn Ali." Before the death of Imam Reza, Ma’mun stood next to his pillow (head side). Imam Reza opened his eyes and said to him, "have a good conduct with my son Muhammad al-Taqi for his death and yours are close to each other and do not have much distance in between." The body of Imam Reza was buried in Mashhad. This occurred when Imam Reza was 55 years of age. Following the death of Imam Reza a revolt took place in Khorasan. Al-Ma’mun wept and beat upon his head to show that he was a mourner. But a large number of people knew that al-Ma’mun himself was the killer of Imam Reza. A wave of despises and noise awn against al-Ma’mun. So that he did not allow the funeral to be carried out for a day and a night. Because, he was afraid that the disturbance may expand and the angry hostile and flared up masses may annihilate all the set ups and organizations. This is the reason why he sent a few men among the people to exhibit and shows the death of Imam Reza as a natural one and tells them that al-Ma’mun did not have a hand in it. But for all he did, he could not get himself acquitted off and prove his innocence. At last, day-by-day he became more and more worthless and despicable in the eyes of the people, until he died in a very bad shape.
source: Tahoor Encyclopedia