Musnad Abu Ya'la al-Mawsili 129:129

Zuhayr narrated to us, Yazid bin Harun narrated to us, Humayd informed us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The Dajjal is wiped of the eye, upon it is a thick film. Written between his eyes is 'K-F-R'."