When al-Ma'moon was unable to bear Imam ar-Redha (a.s) any longer, he decided to assassinate him. He invited him and offered him a cluster of grapes in which he had inserted some poison. He said to him, “O son of the messenger of Allah, I have not seen grapes better than these.”
Imam ar-Redha (a.s) replied, “There may be grapes in Paradise better than these.”
Al-Ma'moon asked Imam ar-Redha (a.s) to eat from those grapes but the Imam was reluctant and said to al-Ma'moon, “Would you excuse me from it?”
Al-Ma'moon scolded and shouted at him, “You must eat them. What prevents you from eating? Are you accusing us of something?”
He then forced Imam ar-Redha (a.s) to eat the grapes. Imam ar-Redha (a.s) ate three berries and then threw the cluster away. The poison acted upon him at once and he got up to leave their meeting. Al-Ma'moon asked him, “Where are you going?”
Imam ar-Redha (a.s) glanced at him and said in a sad and trembling voice, “To where you have directed me.” He meant to death.
The poison reacted in his body and the pains of death attacked him. Al-Ma'moon sent a messenger to Imam ar-Redha (a.s) asking, “What would you recommend me to do?”
Imam ar-Redha (a.s) said to the messenger, “Tell him: he recommends you not to give someone a thing that you may feel regret for.” Imam ar-Redha (a.s) meant the position of heir apparent with which al-Ma'moon had entrusted him and had bound himself by it before Allah and the nation and then he broke his trust.
Imam ar-Redha (a.s) turned to Abussalt and said, “O Abussalt, they did it.” He meant his assassination was wrought by al-Ma'moon.
Imam ar-Redha (a.s) began suffering from the bitter pains of the poison. His intestines were cut and his insides melted. Death was quickly approaching the imam to extinguish that shining flame which had lit the intellectual and social life in the world of the Arabs and the Muslims. Imam ar-Redha (a.s), even in that critical ordeal, was busy in the remembrance of Allah. The bitter pains of that severe death did not prevent him from mentioning his Lord. He breathed his last while praising and glorifying Allah. His great soul ascended to its Creator as the souls of the prophets and imams had done before him, surrounded by the angels and the contentment of Allah. The soul of Imam ar-Redha (a.s) ascended to Allah the Merciful after he had carried out his great, reformative mission of defending the religion of Allah and protecting His principles and goals.
Reference : The Life of Imam Muhammad Al-Jawad