Rayya b. Salt once asked Imam Reza: "Are you Sahib al-Amr (Master of the Undertaking)?" The Imam (a.s) said:
“Yes, I am the Master of the Undertaking. However, I am not that Master of the Undertaking who will fill the earth with justice and equity. How can I be that Master of the Undertaking when you are witness to the prevalent weakness and helplessness? The promised Qaim will be old in age but young in appearance when he emerges. He will be so powerful and strong that if he extended his hand to the most huge tree it would fall uprooted.
And if he screamed among the mountains the rocks would be crushed into pieces. The rod of Moses and the seal of Solomon are with him. That person will be my fourth descendant. God will keep him in occultation for as long as He deems necessary. Then, He will cause him to appear, and through him God will fill the earth with justice and equity as it is filled with tyranny and oppression!”
Bihar al-Anwar ~ Volume: 52, Page: 322