The Knowledge of Imam Reza (as)

Al-Ma'moon had asked the leading scholars of the Islamic districts to come to Khurasan to test Imam ar-Redha (a.s). They argued with him on different philosophical, theological and medical questions and on other branches of knowledge. The scholars left Imam ar-Redha (a.s) afterwards with complete belief in his imamate. They began spreading his virtues and sciences. When al-Ma'moon learned that, he asked Muhammad bin Amr at-Toossi to drive people away from meeting with Imam ar-Redha (a.s).[2] Abussalt al-Harawi exposed this matter in his response to Ahmed bin Ali al-Ansari’s question of “how was al-Ma'moon pleased with killing ar-Redha in spite of his honoring and loving him and his having made him his heir apparent? 
Abussalt replied, “Al-Ma'moon honored and loved Imam ar-Redha (a.s) because he was aware of his virtues which were also known to the people, and he appointed him as his heir apparent to make people think that Imam ar-Redha (a.s) wished for this worldly life and its pleasures and would consequently be disrespected by the people, but when his virtue and respect among people grew even more, he (al-Ma'moon) sent for theologians from different countries hoping that one of them might defeat Imam ar-Redha (a.s) and then he would be disrespected by the scholars and that would spread among the public and they would then turn their backs on him. But, every opponent from the Jews, the Christians, the magi, the apostates, the Brahman, the atheists, the Dahriyya, and every opponent from the dissenting Muslim sects was defeated by Imam ar-Redha (a.s) with his clear evidences. People began saying, ‘By Allah, he is even worthier of the caliphate than al-Ma'moon’ and the newsmen reported that to him (al-Ma'moon) and he became very angry and envious of that.” ( Uyun Akhbar ar-Redha, vol. 2 p.239.)

Reference : The Life of Imam Muhammad Al-Jawad