The story is as follows:
Al-Ma’moon al-Raqi said: “I was with my Master al-Sadiq when Sahl ibn al-Hassan al-Khorasaani entered, so he said his greetings of peace then sat and said to him: “O son of the Messenger of God, may compassion and mercy be upon you. You are from the Ahlulbayt and of the Imams, what prevents you from claiming your rights, while you have 100,000 Shias in front of you and ready to strike with the sword?” Imam al-Sadiq (as) said to him: “Sit O Khorasaani, may Allah protect you”.Then al-Sadiq said: “O Hanifa turn on the oven”, so he turned it on until the burning made the coal white with light.
Then he said: “O Khorasaani, get up and sit in the oven.”Khorasaani replied: “O my Master, O son of the Messenger of God, don’t punish me with fire, exempt me!” so the Imam exempted him.Whilst this was happening Haroon al-Makki entered with his slippers in his hand, and he said: “Peace be upon you O son of the Messenger of God”, Imam al-Sadiq looked at him and said: “drop your slippers from your hands and sit in the oven.”
Al-Makki dropped the slippers and then sat in the burning oven, and the Imam (as) continued to speak in depth to Khorasaani about a hadith as if he was witnessing it, then the Imam said: “O Khorasaani, look what is in the oven.”Khorasaani went to see and found al-Makki inside the oven sitting cross-legged, and then he got up from the oven and greeted us. The Imam (as) then said to Khorasaani “How many do you find in Khorasaan like this one (like al-Makki). Al-Khorasaani replied: “By Allah, not even one.”The Imam (as) replied: “By Allah, not even one.” then he said: “I am living in a time where we cannot find five (true) supporters for us, and we know of the time (we should rise).”
Manaaqib ibn Shahrashoob, Volume 4, Page 257-258.
Bihaar al-Anwaar, Al-Majlisi, Volume 47, Page 123-124.