Imam Reza (as) - Imamat

Mohammed b. Sinān has narrated: [I (i.e. Mohammed b. Sinān) visited Abū al-Hasan (Mūsā) the year before he went to Iraq. ‘Ali, his son, was sitting in front of him. He looked at me and said:] “Mohammed, this year there will be some disturbance but do not be worried by that.”
“My Allah make me your ransom, what will that be?” I asked, for he had filled me with anxiety.
“I will go to this despot,” he answered, “but he will not receive any evil from me nor from the son who will be after me.”
“May Allah make me your ransom, what will happen?” I asked.
“Allah leads astray the oppressors,” he told me, “and Allah does what He wishes.”
“Whoever wrongs the right of this son of mine and denies his Imāmate after me is like those who wronged ‘Ali b. Abū Tālib, peace be on him, and denied his right after the Apostle of Allah, may Allah bless him and his family,” he replied.
“If Allah supports me in my life, may I submit to his right and declare (your belief in) his Imāmate,” I said.
“True, Mohammed,” he said, “Allah will support you during your life and you will submit to his right and declare (your belief) in his Imāmate, and the Imāmate of the one who will be (Imām) after him.”
“Who will that be?” I asked.
“His son Mohammed,” he answered.
I said: “(I give) consent (ridā) and submission to him.( Uyūn Akhbār al-Ridā, vol. 1, p. 26.)

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