The Book Pertaining to the Turmoil and Portents of the Last Hour (Kitab Al-Fitan wa Ashrat As-Sa`ah)
Muslim :: Book 41 : Hadith 7008
Anas b. Malik reported that Allah's Messenger (may peace be upon him) said: There would be written three letters k. f. r., i. e. Kafir, between the eyes of the Dajjal.