Al-Riḍā himself recounts who was responsible for the martyrdom of his father al-Kāẓim

From Imāmī (Twelver Shīʿī) sources it is established that the death of Imam Mūsā ibn Jaʿfar and his overthrow occurred at the hands of Hishām ibn al-Ḥakam¹, with chains of transmission considered sound according to their standards. Among this is what al-Kashshī narrates with a chain whose route is authentic based on Imāmī principles:

From Abū al-Ḥasan al-Riḍā (peace be upon him), who said:
“Did you not have, in Abū al-Ḥasan [al-Kāẓim] (peace be upon him), a lesson? Do you not see the state of Hishām ibn al-Ḥakam?! It was he who did to Abū al-Ḥasan what he did; he said things about him and informed them. Do you think God will forgive him for what he committed against us?”
(Rijāl al-Kashshī, 2/561)

Indeed, al-Kashshī also narrates the condemnation by Imam al-Kāẓim (peace be upon him) of Hishām ibn al-Ḥakam regarding his blood. He said:

From ʿAbd al-Raḥmān ibn al-Ḥajjāj, who said: Abū al-Ḥasan [al-Kāẓim] (peace be upon him) said:
“Go to Hishām ibn al-Ḥakam and say to him: Abū al-Ḥasan says to you: Would it please you to be a شریک (partner/accomplice) in the blood of a Muslim man? If he says no, then say to him: Why, then, did you become a partner in my blood?!”
(Rijāl al-Kashshī, 2/562)

— There is also a narration that his nephew Ismāʿīl informed against him; if that were established, it would be a grave sin. However, according to the Imāmī methodology, an authentic report from an infallible takes precedence over others.
— Then this Hishām ibn al-Ḥakam: he is the Imam of the Imāmīs, their shaykh and leading authority, and among them the “Ark of Noah”!

May God grant you success.
O God, send prayers and peace upon Muḥammad and upon the family of Muḥammad

Translation of Ustadh Kadhim Al Zaydi (May Allah reward him)

Footnotes

¹ – Hisham bin Al-Hakam:

One of the Ashab al-Ijma and among the brightest early Imami theologians.

Initially a servant of Yahya bin Khalid Al-Barmaki, later part of Harun Al-Rashid’s inner circle.

Mentioned in Mu‘jam Rijal al-Hadith by Sayyed Al-Khoei.