The manifestation of Imamate was slowly approaching the city of Marw. Excitement and anticipation for the Imam's arrival filled the city. Ma'mun(1), the Abbasid Caliph, and his companions went ahead to greet the Imam; and Ma'mun was the first to welcome Imam Reza (A.S.). As Ma'mun guided the Imam towards his accommodation, he continuously tried to flatter the Imam and act as a true devotee of Imam Reza (A.S.).
Two buildings resided in the middle of a garden, and a narrow path connected the two building together. It was a twin building that had been built for Ma'mun to live in one of the buildings, and which the Imam stayed in the other.
Ma'mun went in the presence of the Imam and deceitfully said: "O Son of the Messenger of Allah! I am aware of your virtue, knowledge, and holiness. I believe you are more deserving than me to the position of Caliphate. So, please allow me to resign and offer you the responsibility; and allow me to be the first who pledges his allegiance to you, the new Caliph."
Imam Ali ibn Musa (A.S.) was aware of the evil intentions Ma'mun held. Hence, his reply to Ma'mun was one that instead condemned Ma'mun of his intentions. As a result, Ma'mun decided to use another plan, which was to put Imam Reza (A.S.) under pressure.
Imam Reza's (A.S.) reply to Ma'mun's offer was: "If Caliphate was your right, and Allah chose this position for you, you do not have the right to offer this responsibility to another. You cannot resign from the position Allah has assigned upon you. Nevertheless, if Caliphate was not your right, then you do not have the authority to offer me that which is not yours."
A question may arise for some on this matter that if Ma'mun had offered to withdraw, why did Imam Reza (A.S.) refuse to take control of Leadership and Caliphate? And in such a situation who should become the Leader of an Islamic society?
The truth is that Ma'mun did not intend to fully withdraw from the government. He had planned to stay beside Imam Reza (A.S.) and involve himself in many issues within the government. In other words, he wanted Imam Reza (A.S.) to be a leader on the surface, while Ma'mun could use the Imam as cover to continue his oppressions and tyranny.
Imam Reza (AS)
On the other hand the administration team and all the key positions of government at that time were made up of people who had similar ill-intentions like Ma'mun. They were clearly against Imam Reza's (A.S.) method for governing the society. Therefore, with such a team, leading the Islamic society would be impossible.
Moreover, Ma'mun's offer to withdraw was only a strategic move to destroy the image of Imam Reza (A.S.) in the Islamic society. Ma'mun wanted to show people that even when the infallible Imam gains power, he would put his piety aside and hold tight to the position. If it were not for the ill-intention of destroying the reputation of the Imam, Ma'mun was in no way willing to offer leadership to another. He was so dependant on his own leadership that he had killed thousands of people, including his own brother just to gain power.
But in the case that Ma'mun was truly willing to resign along with his whole administration team, providing the grounds for the government of Imam Reza (A.S.), then certainly this divine responsibility would have been put on the shoulders of Imam Reza (A.S.). He would rejuvenate justice and honesty like his great grand father Ali, Amir al-Mu'minin (A.S.). Imam Reza (A.S.) would take the government of Prophet Muhammad (S.A.W.) and Amir al-Mu'minin (A.S.) as a model for governing the Islamic society. (Selection taken from the book "The Role of Imam Reza's (A.S.) Leadership", by Ayatullah Sayyid Muhammad Shirazi)
1- Abu Jafar al-Ma'mun ibn Harun (786 - 833) was the 7th Abbasid caliph who reigned from 813 until his death in 833. Harun al-Rashid, Ma'mun's father, had ordered that Amin, brother of Ma'mun, be his successor and Ma'mun be the governor of Khurasan, and Ma'mun was to succeed Amin after his death. However, since Amin ignored his father's command, a conflict arose between the brothers in which Ma'mun killed Amin. He then continued to rule the empire from the city of Marw in Khurasan. He was well known to be clever, politic, and knowledgable. In an attempt to declare his Caliphate to be valid, Ma'mun forced the eighth Imam, Ali al-Reza (A.S.), to be his heir; however, he eventually poisoned the Imam and martyred him.
source : www.tebyan.net