Sunan Abi Dawud, Hadith 4303

Shu’bah said in his version: “the letters k, f, r” (are on his forehead).

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، عَنْ مُحَمَّدِ بْنِ جَعْفَرٍ، عَنْ شُعْبَةَ، ك ف ر ‏.‏

sunnah.com

also referenced under: Hadith 4317 (DarusSalam)