The Eid Prayer

Al-Ma’mūn asked Imām al-Ridā, peace be on him, to say  the ‘Īd prayer before the people and to deliver a sermon after the prayer, that the hearts of the populace might become assured through that and recognize his excellence, but the Imām refused to respond to him and said to him: “You have come to know about the conditions made between us; I do not interfere in this affair; therefore, exempt me from saying the prayer before the people.” “I only intend by that that the people’s heart should be assured and that they should know your great merit,” replied al-Ma’mūn.
Al-Ma’mūn insisted on that, so the Imām was forced to respond to him, but he made it a condition that he should go out just as Allah’s Apostle, may Allah bless him and his family, and his grandfather the Commander of the faithful, ‘Ali b. Abū Tālib, peace be on him, did.
“Go out as you wish,” replied al-Ma’mūn. Then he ordered the military commanders and the rest of the people to receive Imām al-Ridā, peace be on him.
The people went out waiting for the Imām in the roads and on roof-tops. As for the military commanders, they were ready and wore the best uniform. When the sun rose, the Imām washed and put on a white turban. One end of it he made hang on his holy breast and the other between his shoulders. He took his staff in his hand and said to his retainers: “Do whatever I do.” Then he went out in that humble state, raised his head towards heaven, and exclaimed four times: “Allah is great (Allahū akbar)!” Then he stood at the door and exclaimed four times: “Allah is great (Allahū akbar)!” Then he said: “Allah is great, for He has guided us! Allah is great, for He has given to us of the cattle quadrupeds! Praise belongs to Allah, for He has tried us!”
The earth shook with saying ‘Allah is great!’ The people walked like waves and said at the top of their voices: “Allah is great!” The appearance of the Imām, peace be on him, reminded them of that of his grandfather the Messenger, may Allah bless him and his family, who developed the intellectual life on earth. So they understood the deviation of those kings who ruled them with oppression and tyranny.
The great Imām, the peace of Allah be on him, walked bare-foot, stopped after each ten steps, and said four times: “Allah is great!” The people imagined that the sky, the earth, and the walls were answering him. As for Marū, it shook with weeping and clamor. Al-Ma’mūn heard of that and he became frightened and terrified. So al-Fadl b. Sahl hurried to him and said to him: “O Commander of the faithful, if al-Ridā reaches the place of prayer for the festival, the people will break out in rebellion. So send instructions to him to go back.”
Accordingly, al-Ma’mūn sent one of his policemen to the Imām to ask him to go back, so he, peace be on him, called for his boots and put them on. Then he went back without saying the prayer before the people ( Usūl al-Kāfi, vol. 1, p. 189-190. 'Uyūn Akhbār al-Ridā, vol. 2, pp. 150-151. Al-Manāqib, vol. 4, pp. 371-372. Kashf al-Ghumma ) 

Reference :The life of Imām'Ali Bin Mūsā al-Ridā