The story of Mamoun's (ruler of the time) first meeting with the young son of Imam Ali Al-Reza (AS) is interesting
One day, when the young Imam was only nine years old, the caliph Mamoun went out hunting. The Imam was standing silently by the road side where some children were playing. The caliph's entourage came that way. Seeing the soldiers of the caliph all the children ran away, but the young Imam remained standing at his place.
Noting this, Mamoun stopped his carriage and asked, "Young man, why did you not run away like the other children?"

The Imam replied calmly, "Neither had I committed a crime, nor was I blocking the way. Why should I have run away or be afraid?  And I also know that you will not cause any unnecessary trouble when your way is not blocked."
Mamoun was surprised with this mature reply and asked, "What is your name?"
"Muhammad," came the reply. "Whose son are you?" asked Mamoun. "Son of Ali Ibn Musa (AS)."

Mamoun rode on. During his hunt the hawk returned to him with a fish in its beak. Mamoun was surprised. He returned back toward the city. Once again, he found children playing on the same spot, who ran away seeing the caliph's soldiers, except this young man who said he was Muhammad son of Ali Ibn Musa (AS) who remained where he was.

Mamoun hid the fish in his palm, stopped his carriage near the Imam and said, "Tell me, what is there in my fist?" The Imam replied, "God created clouds between earth and sky. The hawks of kings sometimes catch fish from there and bring it to the Kings. They hide it in their fist and ask a member of the Ahlulbayt of the Prophet, Tell me what is there in my fist."  Mamoun said, "Truly, you are the worthy son of Imam Ali Al-Reza (AS)" (Bihar-al Anwar, Majlisi).

Mamoun took the young Imam with him, and let him live in a nearby house next to the Royal Palace.  It was during this stay of the 9th Imam nearby that Mamoun had the chance of seeing him closely and gaining an insight into the intellectual abilities of this pious household.

ICA - Project Enlightenment