Musnad Ahmad 774:19

Aswad narrated to us, Sharik narrated to us, from al-A’mash, from Khaythama, from Ibn Ma’qal, from ‘Adi ibn Hatim, who said: The Prophet, peace and blessings of Allah be upon him, said: "Fear the Fire." He said: Then he turned his face away until we thought he was looking at it. Then he said: "Fear the Fire," and he turned his face away. He said: He said it two or three times: "Fear the Fire, even if with half a date; and if you do not find [that], then with a good word."