Imam (A) was brought up under the care of his father for 4 years. When the Abbaside Caliph Ma'mun forced Imam ar-Riza (A) to leave Madina for Iran, he knew that he would never see his young son again. He therefore declared Imam (A) as his successor, so that the people of Madina would be in doubt as to who was the next Imam. In 203 A.H. the cursed Ma'mun poisoned Imam Ali ar-Riza (A) and thus Imam (A) took over the duties of Imamat when he was only 8 years old.

In spite of his minor age, he demonstrated the intelligence and wisdom of his forefathers. He was bold, learned, used to forgive people and was tolerant. He was famous for his hospitality and courtesy to all and for his help to the poor, orphans and the needy. He lived a simple life and worked tirelessly to guide people to the right path.

Ma'mun received reports of the brilliance of Imam (A) in debating in matters of Fiqh, Hadith and Tafsir of the Holy Qur'an. He wanted to confirm these reports for himself so he had Imam (A) brought to his capital in Baghdad.

Although Ma'mun had shown in many instances his true enmity towards Ahlul Bayt (A) and their Shia, he welcomed Imam (A) for his own ulterior motives. He did not doubt that the young man was the Imam of the time despite his age, and he knew that the Shia would follow the rulings of Imam (A) without hesitation. In order to protect himself and the future of the Abbaside empire, Ma'mun wanted to marry his daughter Ummul Fazl to Imam (A).

The elders of Bani Abbas were very disturbed when they learnt of Ma'mun's intentions. A group of them came to Ma'mun with a view to dissuade him from this marriage. Ma'mun rejected their plea, stating that Imam (A) was a true successor to his father and commanded the same virtues and excellence in spite of his tender years. He further stated that the best scholars of the Islamic world could not hope to compete with Imam (A). This gave the elders an opportunity to prove him wrong and they decided to appoint Yahya bin Aksam, the greatest scholar and juror of Baghdad, to debate against Imam (A).

Ma'mun agreed to the contest and organised a large gathering in his court, in which 900 scholars and learned men were invited to participate. News of the contest spread quickly and people wondered how a young child could contest against the veteran judge who was famous for his knowledge.

When the court was assembled, Yahya confidently asked Imam (A), "What is your verdict about a man who indulges in hunting while in the state of Ehram?"

Imam (A) immediately replied, "Your question is vague and misleading. Perhaps you could clarify it by mentioning whether the man hunted within the precincts of the Holy Ka'ba or outside; whether he was literate or illiterate; whether he was a slave or a free man; whether he was Baaligh or not; whether it was for the first time or he had done it previously; whether the prey was a bird or some other creature; whether the prey was small or big; whether he hunted in the day or at night; whether the hunter repented for his action or persisted in it; whether he hunted secretly or openly and whether the Ehram was for Umrah or Haj. Unless these points are explained, no specific answer can be given to this question".

Yahya was staggered at listening to these words and the audience was dumbfounded. He acknowledged the superiority of Imam (A) and humbly requested to be told the answer. Imam (A) then gave the different verdicts for each of the possibilities listed above.

Ma'mun was overjoyed at this display of the knowledge of Imam (A) and in the same gathering, he wedded his daughter Ummul Fazl to him. After staying in Baghdad for a year, Imam (A) returned to Madina.

His marriage with Ummul Fazl was not peaceful and she was a source of constant difficulty to him. She was also jealous because he married Bibi Summana Khatoon (A), who later became the mother of the ninth Imam (A).

In Madina Imam (A) once again gave the people true guidance and taught the religion of Islam freely. People from far and wide would come to Madina to seek knowledge and truth from him.

After Ma'mun died Mu'tasim Billah came to the throne. He was a man who hated the Ahlul Bayt (A) deeply. He knew that it was difficult to act against Imam (A) in his home city of Madina, so he called him to Baghdad.

In the same year the cursed man got Imam (A) poisoned. Imam (A) passed away when he was only 25 years old, and is buried beside his grandfather, Imam Musa al-Kazim (A), at Kazmain in the suburb of Baghdad.