Answer
The death of Musa al-Kazim, the seventh Shia Imam, is speculated to have been instigated by Abbasid Caliph Harun al-Rashid. The Caliph had become concerned that Musa al-Kazim would challenge his authority and sought to have him killed. Musa al-Kazim was poisoned in 799 CE, and the Caliph was accused of ordering his death.