Wherever Imam Riḍā (P.B.U.H.) stopped, during his journey toward Marv, he caused for many blessings upon the people. At one point on his way, he entered the city of Neyshābūr. A large crowd of people gathered to welcome his entrance. Meanwhile two learned men who had recorded many prophetic traditions came with scholars of ḥadith, took the halter, and said, “You, the great Imam for the sake of your innocent fathers and your great grandfathers, show us your shining face and narrate us a tradition from your fathers and your great grandfather, the Messenger of Allah (P.B.U.H. & H.H.), to be as a reminder for us”. Imam (P.B.U.H.) ordered to stop. After a while the crowd got silent and Imam (P.B.U.H.) dictated this tradition word by word from his honored father which was narrated from his pure grandfathers and from the Messenger of Allah (P.B.U.H. & H.H.) and from Gabriel and from Almighty Allah that, “The word of La Ilaha Illa Allah is my fortress (then whoever says this has entered my fortress) and whoever enters my fortress will be saved from my punishment”. Imam (P.B.U.H.) paused for a while and then said, “But on some conditions and I am one of its conditions.” 
This tradition indicates that one of the necessities of saying the word of  ,La Ilaha Illa Allah which is to strengthen the principle of Divine unity, is confirming his Imamate, obeying, and accepting his words and deeds that has been specified by the Almighty Allah. In fact Imam (P.B.U.H.) believes the Oneness of Allah (Tuḥīd) as a condition of being safe from Allah’s punishment and the acceptance of Leadership and Mastership (Imamate and Wilāya) as the condition of believing in Oneness of Allah.
 Maʿmūn had no choice but to accept this condition.  After that Imam (P.B.U.H.) raised his hands and said, “O Allah! You know that they opposed me under duress and I chose it by force. So please do not take me to task just as Your two prophets, Joseph and Daniel, when they accepted the leadership position of the kings of their time. O Allah! There is no vow except Yours and no Mastership except of Yours. So please help me to establish Your religion and to follow Your Prophet’s (P.B.U.H. & H.H.) custom.  Indeed, what a good Master and a good Assistant You are”.