The Messenger of Allah ﷺ said, "Dajjal will have one eye effaced, and between his eyes will be written 'disbeliever'." He then spelled it out, "K-F-R, which every Muslim will be able to read."
قال رسول الله ﷺ: الدجال ممسوح العين مكتوب بين عينيه كافر، ثم تهجاها ك ف ر يقرؤه كل مسلم.
Muslim narrated it in the Book of Trials and Signs of the Hour (2933:103), from Zuhayr bin Harb, who said, Affan narrated to us, who said, Abdul-Warith narrated to us, from Shuayb bin Habhab, from Anas bin Malik, who said, and he mentioned it.