Imam Musa bin Jafar (A.S.) said: "A wise person would never promise something that he is incapable of doing; he would never hope something for which he would be reproached for; and would never take action for something that he fears he may fail in."
One year Harun al-Rashid (the Abbasid caliph) entered Medina before going to Hajj. It was not long after his arrival that he ordered his companions to arrest Musa ibn Jafar (A.S.). They arrested Imam Musa ibn Jafar (A.S.) while he was in the mosque, and brought him to Harun. Harun had already prepared two rides. He chained the Imam and made him get on one of the camels in a manner that he could not be seen. After that, Harun ordered to send one of the camels towards Kufah and the other towards Basrah. Imam was on the camel that went towards Basra. At that time, Isa was the commander of Basra; so they delivered the Imam to Isa.
Isa kept the Imam in jail for a year. At last Harun wrote him a letter and asked him to kill the Imam. Isa gathered some of his relatives and consultants, and asked their opinions about killing the Imam. They advised Isa not to do so, and to ask Harun to excuse him from killing the Imam. Then, Isa wrote a letter to Harun in which he stated: "It has been a while that Musa ibn Jafar is in my jail; I have been watching him all the time and I have put some guards to look over him; however, he has done nothing but worship. I assigned someone to listen to his prayers, but he was never heard cursing us; and as for himself, he asked for nothing but forgiveness and mercy from Allah. I request that you send someone to take Musa ibn Jafar from me; otherwise, I will set him free since I can not keep him in jail anymore."
In this regards, some of the guards whom Isa had appointed to watch over the Imam have mentioned that they would constantly hear the Imam saying: "O Allah! You are aware that I had asked for a private place to worship You, and You have provided me with such a place; praise shall only be to You".
After that, Harun sent Imam Musa al-Kazim (A.S.) to Fazl ibn Rabi', who was one of his ministers, and ordered him to kill the Imam; however, he refused to do so. Then, it was Fazl ibn Yahya's turn to refuse killing the Imam. Finally, Harun gave his order to someone who had no mercy in his heart. He sent a messenger to Sendi ibn Shahak (a Jew who was the chief police of Baghdad), and ordered him to kill the Imam. Sendi ibn Shahak poisoned some food and brought it for the Imam in his jail... After years of suffering from pain and hardships in jail, Imam became severely ill from that poison, and faced his martyrdom after three days.
(Selection taken from:" Irshad" by Sheikh Mufid, translated by Sayyid Hashim Rasouli Mahallati, with some changes)
source : www.sibtayn.com
