Musnad Ahmad 1140:1

Hashim ibn al-Qasim and Abu Abd al-Rahman al-Muqri narrated to us, they said: Sulayman ibn al-Mughirah narrated to us, from Humayd ibn Hilal, from Abu Rifa’ah al-Adawi, who said: I came to the Prophet, may Allah bless him and grant him peace, while he was delivering a sermon. I said: "A stranger man has come to ask about his religion; he does not know what his religion is." He said: The Prophet, may Allah bless him and grant him peace, turned toward me and left his sermon. Then a chair was brought, its legs were of iron, and the Messenger of Allah, may Allah bless him and grant him peace, sat upon it. Then he turned to me and began teaching me from what Allah had taught him. Then he came to his sermon and completed the rest of it. Abu Abd al-Rahman said in his narration: Humayd said: He said: I think he saw black wood and supposed it was iron.