Tradition

Harun Rashid declared when his son Mamun asked him about the reason why he admired and magnified Imam Moosa Kazim (a.s.) – My son, this is the Imam of the people, the Proof of Allah’s Mercy to His creation (Hujjatullah) and His Caliph among His servants. I am outwardly the Imam of the masses by force and through oppression, while Moosa b. Jafar (a.s.) is the Imam in reality. And surely he, by Allah, is more worthy of being the successor of the Messenger of Allah (s.a.w.a.) as the caliph than I me and anyone else among all the people. And by Allah, if you yourself attempt to take such caliphate from me, I will take it away from you even if that means gouging your eyes, for power is blind.
Yanaabi al-Mawaddah vol 3 p 32