Musa b. Jaʿfar b. Muhammad b. ʿAli b. Husayn b. ʿAli b. Abi Talib, Abū'l-Ḥasan al-Hāshimī. He was also known as al-Kāẓim. He was born in the year 128 or 129 of the Hijra. He was a person devoted to much worship and possessed of noble character (murūwwa). If he heard that someone claimed to have been harmed by him, he would send that person gold and gifts. He had forty children, both male and female. On one occasion, a slave gifted him some bread with fat. In response to this act, he purchased the slave and also bought the field in which the slave worked for 1,000 dinars, then freed the slave and donated the field to him.
The caliph al-Mahdī summoned him to Baghdad, and upon his arrival, imprisoned him. One night, al-Mahdī saw ʿAlī (peace be upon him) in a dream, and ʿAlī addressed him: "O Muḥammad! If you turn back, will you not be expected to spread corruption on earth and sever ties of kinship?" (al-Muḥammad 47:22).
Upon seeing this dream, al-Mahdī awoke in a panic and ordered Musa b. Jaʿfar to be released from prison. After he was brought to him at night, al-Mahdī seated him by his side, embraced him, and showered him with compliments. However, he made Musa b. Jaʿfar promise that neither he nor any of his sons would rebel against him. Musa b. Jaʿfar replied: "By Allah, I am not one to do such a thing, nor have I ever intended it." Al-Mahdī said: "You have spoken the truth," and ordered that he be given 3,000 dinars and sent to Medina. He was sent off in the morning. Musa b. Jaʿfar remained in Medina until Hārūn al-Rashīd assumed the caliphate.
When Hārūn al-Rashīd went on pilgrimage, he entered the mosque of the Messenger of God (peace and blessings be upon him), and when he reached his grave, Musa b. Jaʿfar al-Kāẓim was with him. Hārūn al-Rashīd greeted the Messenger of God (peace and blessings be upon him) saying: "Peace be upon you, O Messenger of God! O cousin." Musa b. Jaʿfar greeted him saying: "Peace be upon you, O my father." Upon this, Hārūn al-Rashīd said to him: "This is boasting, O father of Husayn," and from then on began to harbor resentment toward him. Finally, in the year 169 of the Hijra, he summoned him to Baghdad, where he imprisoned him for a long time. While in prison, Musa sent the following letter to Hārūn al-Rashīd:
"Now then, O Commander of the Faithful, whenever a day of affliction passes over me, a day of blessing and prosperity passes over you in return. Thus we shall both reach the Day of Resurrection. On that day, those who followed falsehood and acted unjustly will be the losers."
Musa b. Jaʿfar died in Baghdad five days before the end of the month of Rajab in that year. His grave is in a well-known location there.