The Fork from Ismail
The Twelver-Ismaili split occurred at the death of Ja’far al-Sadiq (765 CE). Those who held that the Imamate passed to Ismail (who predeceased his father) — and through him to Ismail’s son Muhammad ibn Ismail — became Ismailis. Those who held it passed to Ja’far’s surviving son Musa became the majority Twelver Shia tradition. Musa al-Kazim did not seek this controversy; he simply transmitted the knowledge of his father and trained students under increasingly difficult conditions.
Imprisonment Under Harun al-Rashid
The Abbasid Caliph Harun al-Rashid (r. 786-809 CE) viewed the line of the Imams as a political threat — any descendant of Ali who commanded popular love and religious authority was a potential focus for revolt. Musa al-Kazim was arrested, imprisoned in Baghdad, transferred between prisons, and kept in isolation for years.
During this time, his legal opinions were transmitted secretly to followers who continued his work. He maintained his prayer, his scholarship, and his character. Accounts describe him praying voluntarily through the night in his cell.
He died in prison in 799 CE. The circumstances — physical decline after years of imprisonment — and later reports of poisoning made his death a martyrdom narrative in Twelver tradition.
The Title Al-Kazim
Kazama al-ghayza — to swallow one’s anger, to restrain fury that is legitimate but better suppressed for a higher purpose. The Quran praises this quality (3:134): “Who spend [in the cause of Allah] during ease and hardship and who restrain anger and who pardon the people.” Musa al-Kazim embodied this over a lifetime, including under imprisonment — which is why the title stuck.
See also: Ahl Al Bayt, Seerah Jafar Al Sadiq, Seerah Ali Zayn Al Abidin, Fitna Islamiyya, Quran Sciences, Tafsir Overview