The continuation of the general movement of the Imams during the time of Imam Kadhim (Musa ibn Ja'far AS) is very venturesome and exiting. In my opinion, this era marks the apogee of the struggles of the infallible Imams. Unfortunately we do not have a proper, illuminating report on the life of Imam Kadhim (as).
There are certain events in his life that astonishes human beings. For instance, some traditions indicate that for sometimes the Imam went underground and conducted a clandestine life to rescue his life from the agents of the ruling system. Although Harun's government tried to find him, it could not discover his hideout. The Caliph even tortured a number of people to disclose the hideout of the Imam. This is unprecedented in the lives of the infallible Imams.
There is another tradition about Imam Kadhim (as), similar to which we have not seen about other Imams: Ibn Shahr Ashub narrates a tradition in "Manaqib": "Musa ibn Ja'far arrived in a village of Damascus in disguise while running away." These facts are some sparks in Imam Musa's life, which can explain the reasons for his frequent imprisonment and torture by the ruling system. At the beginning of Harun's caliphate the conditions was not so harsh.
When Harun came to power, he went to Medina and as you have heard he cherished and respected the Imam. Mamun narrated: "Imam Kadhim (as) was coming on a horseback when he arrived the area Harun was sitting. Imam decided to get down, but Harun pledged that the Imam should go to his site mounted on the horse. He respected the Imam and exchanged views with him. When the Imam decided to leave, Harun asked me [Mamun] and Amin to help the Imam mount his horse."
Interestingly, according to this tradition, Mamun says: "my father presented 5,000 or 10,000 dinars or drachmas to everybody, but 200 drachmas to Musa bin Ja'far." This is while he had asked about Imam's condition, and the Imam complained' about his difficulties, harsh condition of life and great number of children.
Such statements and remarks of Imam Kadhim (as) in front of Harun are very interesting and understandable, particularly for those who have experienced precautionary dissimulation during the struggles at our time. Imam tells a person like Harun that his condition is not good and adds that he cannot make both ends meet.
There is no humbleness or loss of face in such statements of the Imam, because you as the combatants at the period of Shah of Iran - have made such remarks during the repression and oppression of the tyranny of Shah. It is but natural for activists to deceit the enemy about their activities, conditions and works.
Given these remarks, Harun should have given the Imam some 50,000 drachmas to overcome his economic difficulties, but he gives the Imam only 200 drachmas. Mamun says: "when I asked the reason from my father, he said, if I had paid him the large amount, possibly he would recruit hundred thousand swordmen of his Shiites and friends to fight against me."
Harun's conclusion was right. I think he had understood the Imam correctly.
Some thinkers argue that Harun's conclusion was based on aspersion against the Imam, but the fact is that he had realized the intention and contention of the Imam. When Imam Musa was fighting against Harun, there were a number of people who were ready to stand on his side, but the Imam could not finance his struggle.
We have observed such armed struggles in the case of the sons and grandsons of the infallible Imams. Definitely, the Imams could mobilize the people better than their sons. Hence, the era of Musa ibn Ja'far (as) is the peak of struggle, which finally leads to his imprisonment.