Imam Musa ibn Jafar (a.s) is known as Kazim. That is because he used to be kind even to those who treated him badly.
There was a man in Medina who used to bother imam Kazim(a.s) whenever he saw the Imam and used to say bad words about imam Ali (a.s). A number of the Imam's companions decided to punish the man but Imam Kazim(a.s) forbade them strongly. Then he asked his followers about him. They replied he had a farm in the outskirts of Medina. Imam Kazim (a.s) rode to the man's farm and saw him in his farm. Imam entered the farm and approached him. The man shouted angrily at imam telling him not to enter the farm and step on the plants. But the Imam continued his way till he reached that man and sat beside him. He shook hand with him and treated him kindly. Then asked the man: 'How much have you spent on your farm to grow plants?'
'One hundred dinars.' The man said.
The imam asked again: How much do you expect to earn?
He said: I don't know the unknown.
The imam said: I just asked how much you expect.
He said: three hundred dinars.
The imam took out a bag in which there was three hundred dinars and gave it to the man and said: this is yours and God give you of your farm what you hope.
The man stood and kissed the imam on the head and asked forgiveness for his misbehavior. The Imam smiled and left him and headed for the mosque. When he got to the mosque he saw the man sitting in the mosque. When the man saw Imam Kazim (a.s) he recited this verse: Allah is most knowing of where He places His message..[1] The followers of the Imam surprised at seeing this change of behavior went to the man and said: you used to say different sort of things. He said: you heard me what I said now. Then he started praying for Imam Kazim(a.s). When the Imam returned home he said to his companions: which was better- what you wanted or what I did?[2][3]
Footnotes:
[1] 6:124, اللَّـهُ أَعْلَمُ حَيْثُ يَجْعَلُ رِسَالَتَهُ.
[2] Fadl ibn Hasan Tabarsi, I'lam al-Wara bi A'lam al=Huda, pp. 306-307.
[3] http://www.aviny.com/occasion/ahlebeit/imamkazem/veladat/86/40dh-kazem/4...