Musnad Ahmad 32:1677

'Affan narrated to us, Hammad narrated to us, he said: Humayd and Shu'ayb ibn al-Habhab informed us, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the Dajjal is one-eyed, and indeed your Lord is not one-eyed. Between his eyes is [written] K-F-R (Kafir), which every believer will read, whether he is literate or illiterate." And Hammad also said: "Written between his eyes."