Imam Mousa a-Kazim (A.S.) is reported to have often said to his sons: "Ali ibn Mousa, your brother, is the learned scholar of the Descendants of Muhammad (S.A.W.); therefore, you may ask him about your religion, and memorize what he tells you for I have heard my father Ja"fer ibn Muhammad more than once saying, `The learned scholar of the family of Muhammad is in your loins. How I wish I had met him, for he is named after the Commander of the Faithful Ali (A.S.)."
Ibrahim ibn al-Abbas al-Suli is reported to have said: "I never saw al-Reza (A.S.) unable to provide the answer to any question he received, nor have I ever seen any contemporary of his more learned than he was.”
Al-Mamoon used to put him to test by asking him about almost everything, and he always provided him with the answer, and his answer and example was always derived from the Holy Quran."
Rajaa ibn Abul-Dahhak, who was commissioned by al-Mamoon to escort al-Reza (A.S.) to his court, said: "By God! I never saw anyone more pious than him nor more often remembering God at all times nor more fearful of God, the Exalted. People approached him whenever they knew he was present in their area, asking him questions regarding their faith and its aspects, and he would answer them and narrate a great deal of hadith from his father who quoted his forefathers till Ali (A.S.) who quoted the Messenger of God (S.A.W.). When I arrived at al-Mamoon"s court, the latter asked me about his behaviour during the trip and I told him what I observed about him during the night and during the day, while riding and while halting; so, he said: `Yes, O son of al-Dahhak! This is the best man on the face of earth, the most learned, and the most pious."
Al-Hakim is quoted in Tarikh Nishapur as saying that the Imam (A.S.) used to issue religious verdicts when he was a little more then twenty years old.
In Ibn Maja's Sunan, in the chapter on "Summary Of Cultivating Perfection," he is described as "the master of Banu Hashim, and al-Mamoon used to hold him in high esteem and surround him with utmost respect, and he even made him his successor and secured the oath of allegiance for him."
Al-Mamoon said this once in response to Banu Hashim: "As regarding your reaction to the selection by al-Mamoon of Abul-Hassan al-Reza (A.S.) as his successor, be reminded that al-Mamoon did not make such a selection except upon being fully aware of its implications, knowing that there is no one on the face of earth who is more distinguished, more virtuous, more pious, more ascetic, more acceptable to the elite as well as to the commoners, or more God-fearing, than he (al-Reza, A.S.) is."
Abul-Salt al-Harawi is quoted saying: "I never saw anyone more knowledgeable than Ali ibn Mousa al-Reza (A.S.). Every scholar who met him admitted the same. Al-Mamoon gathered once a large number of theologians, jurists and orators and he (al-Reza, A.S.) surpassed each and every one of them in his own respective branch of knowledge, so much so that the loser admitted his loss and the superiority of the winner over him."
He is also quoted saying: "I have heard Ali ibn Mousa al-Reza (A.S.) saying, `I used to take my place at the theological center and the number of the learned scholars at Medina was quite large, yet when a question over-taxed the mind of one of those scholars, he and the rest would point at me, and they would send me their queries, and I would answer them all."
In his discourse regarding the issue of succession,Mamoon said: "I do not know any man on the face of earth who is more suited (to be heir to the throne) than this man."
Al-Manaqib records the following: "When people disputed regarding Abul-Hassan al-Reza (A.S.), Muhammad ibn "Isa al-Yaqtini said, `I have collected as many as eighteen thousand of his answers to questions put forth to him." A group of critics, including Abu Bakr the orator in his Tarikh and al-Tha"labi in his tafsir and al-Sam"ani in his dissertation and in al-Mu"tazz in his work, in addition to others, have all quoted hadith from him."
After an intellectual discourse with al-Mamoon, Ali ibn al-Jahm said: "Al-Mamoon stood up to perform the prayers ritual and took Muhammad ibn Jafer, who was present there, by the hand, and I followed both of them. He asked him: `What do you think of your nephew?" He answered, `A learned scholar although we never saw him being tutored by any learned man." Al-Mamoon said:`This nephew of yours is a member of the family of the Prophet (S.A.W.) about whom the Prophet (S.A.W.) said:`The virtuous among my descendants and the elite among my progeny are the most thoughtful when young, the most learned when adult; therefore, do not teach them for they are more learned than you are, nor will they ever take you out of guidance, nor lead you into misguidance."
Ibn al-Athir writes: "He (al-Mamoon) discerned the descendants of Banu al-Abbas and Banu Ali and did not find anyone more than him (al-Reza, A.S.) in accomplishments, piety and knowledge."
We do not need the testimony of anyone to convince us of the distinction enjoyed by Imam al-Reza (A.S.) due to his knowledge over all others. Suffices us to review the books of hadith, which are filled with his statements and dictation in various arts which every individual, regardless of the loftiness of his degree of knowledge, became dwarfed upon meeting him, feeling his inferiority and the superiority of Imam al-Reza (a.s.)