After around four years of the Imam’s imprisonment, Haroon had become completely hopeless to break the Imam’s resistance against his government and decided to kill the Imam. He left Baghdad and went to Raqqa, a faraway city in modern day Syria, to portray himself as innocent in the Imam’s martyrdom. He sent his minister, Yahya ibn Khalid Barmaki, to Baghdad with a secret mission to poison the Imam. Yahya was motivated to gain Haroon’s trust because Haroon had been disappointed by Yahya’s son, Fazl ibn Yahya, in how he had honored the Imam during the Imam’s imprisonment in his house. Yahya delivered Haroon’s order to Sendi ibn Shahak and gave him poisonous dates. Sendi served the poisonous dates to the Imam, which led to the Imam’s martyrdom.
Immediately after poisoning the Imam and before his martyrdom, Sendi wanted to portray the Imam’s martyrdom as a natural death to the public. He moved the Imam to a comfortable setting with nice carpets. He then gathered a group of around 80 people from the scholars and the elders of Baghdad in his house. He brought them face to face with the Imam and asked them to testify that the Imam was physically safe and unharmed. He told them, “The people think we are hurting Musa ibn Ja’far but as you see, he is healthy and in comfort, and we have only kept him here until Haroon returns to Baghdad to speak and debate with him.” The Imam unexpectedly began to talk and told them that he had just been poisoned and would pass away in three days. By the Imam’s words, Sendi became nervous and his plot failed.
Three days after the Imam’s poisoning, his soul ascended. He was martyred on the 25th of Rajab, in the year 183 of Hijri, in Baghdad, at the age of 55.

After the Imam’s martyrdom, Sendi still wanted to complete his previously unsuccessful plot of portraying a natural death for the Imam to the people. He gathered more than 50 people who knew the Imam to his house. He showed them the Imam’s body, and asked them to testify that there was no sign of injury or suffocation on his body.
In order to insult the Imam and his Shia followers, Sendi then ordered only four of his soldiers to take the Imam’s corpse to the streets of Baghdad. He asked his soldiers to loudly announce in the streets of Baghdad, “Anyone who wants to see the body of the vicious, son of the vicious, come to us as we are carrying the body of the Imam of the Rafidhi.” Rafidhi was a derogatory term used against the Shias. They then placed the Imam’s corpse on the bridge over the Tigris River for part of a day. The people came to see the Imam’s corpse and witnessed that his body did not have any sign of torture or injury.
Haroon’s uncle, Sulayman ibn Abi-Ja’far, was one of the most influential members of the Abbasid royal family. He was in his palace by the Tigris River when he heard the turmoil and the people’s voices. When he was informed about Sendi insulting the Imam’s corpse, he condemned Sendi’s action, and found it politically unwise and dangerous