From Sahih Muslim (Hadith 2349):
Adi b. Hatim reported that the Messenger of Allah (ﷺ) made a mention of Fire. He turned his face aside and diverted his attention and then said:Guard (yourselves) against Fire. He turned his face and diverted his attention till we thought as if he were (actually seeing it and then said: Protect yourselves against Fire even if it is with half a date, and he who does not find it, (he should do so) with pleasant words. Abu Kuraib did not mention the word: (as if)
Reference: Muslim 2349