Sahih Bukhari Volume 8, Book 76, Hadith Number 568.

Narated By ‘Adi bin Hatim : The Prophet mentioned the Fire and turned his face aside and asked for Allah’s protection from it, and then again he mentioned the Fire and turned his face aside and asked for Allah’s protection from it and said, “Protect yourselves from the Hell-Fire, even if with one half of a date, and he who cannot afford that, then (let him do so) by (saying) a good, pleasant word.”