Musnad Ahmad 32:1450

Yunus narrated to us, Shaiban narrated to us, from Qatadah, who said: Anas ibn Malik narrated that the Prophet of Allah, peace and blessings of Allah be upon him, said: "Between his eyes is written: K-F-R—spelled out—he says: 'Kafir' (disbeliever). Every believer, illiterate or literate, will read it."