There would be written three letters k. f. r., i. e. Kafir, between the eyes of the Dajjal.
Sahih Muslim Book 54, Hadith 126
← Previous Hadith There is never a prophet who has not warned the Ummah of that one-eyed liar; behold he is one-eyed and your Lord is not one-eyed. On his forehead are the letters k f. r. (Kafir).

Sahih Muslim Book 54, Hadith 125 Next Hadith → Dajjal is blind of one eye and there is written between his eyes the word" Kafir". He then spelled the word as k. f. r., which every Muslim would be able to read.

Sahih Muslim Book 54, Hadith 127