Once a Khorasani chief by the name of Sahl bin Hasan came to Imam Jafar ibn Muhammad al-Sadiq (as), and asked him why the Imam wasn't fighting for his rights in spite of the truth being on his side when there were thousands of supporters ready to fight with him.
Imam Jafar ibn Muhammad al-Sadiq (as) with the intention of giving him a practical answer, asked him to sit down and wait. In the meantime, a servant of Imam Jafar al-Sadiq (as) came, and Imam Jafar ibn Muhammad al-Sadiq (as) asked her if she had kindled the fire. She replied, "Yes".
Imam Jafar ibn Muhammad al-Sadiq (as) took Sahl bin Hasan with him and went to the fireplace. The fire was blazing red. Imam Jafar al-Sadiq (as) turned to Sahl bin Hasan and told him to jump into the fire. Sahl bin Hasan trembled and begged forgiveness from Imam Jafar al-Sadiq (as) and asked not to be burnt.
At this moment, Harun Al-Makki one of the sincere companions of the Imam (as) happened to arrive.Imam Jafar as Sadiq (as) told Harun Al-Makki to jump into the fire, and Harun Al-Makki did this at once, without hesitating.
Imam Jafar al-Sadiq (as) then turned to Sahl bin Hasan and started to brief him about the circumstances prevailing in Khorasan, as if he had been there to witness the events taking place.
After a while, Imam Jafar al-Sadiq (as) asked Sahl bin Hasan to go check on Harun Al-Makki. Sahl bin Hasan saw Harun Al-Makki sitting cross legged in the fire safely. Imam Jafar al-Sadiq (as) then asked Sahl bin Hasan how many of his followers were like this man. Sahl bin Hasan replied, "none, master", Imam Jafar al-Sadiq (as) then asked Harun Al-Makki to leave the fire and no burns were seen on him!
Imam Jafar al-Sadiq (as) then explained, "I shall not stage an uprising when I do not have even five sincere companions. (And do remember) we are very well aware as to when we should stage an uprising."
source : sibtayn