Traced to Ka'b al-Ahbar (automatic guess): a report of earlier scriptures, not a hadith of the Prophet ﷺ.
كان يقال: كلب الساعة الدجال، ومن صبر على فتنة الدجال لم يفتن، ولن يفتن أبدا حيا ولا ميتا، ومن أدركه ولم يتبعه وجبت له الجنة، وإذا خلص الرجل وكذب الدجال مرة واحدة، وقال: قد علمت من أنت، أنت الدجال، ثم قرأ عليه بفاتحة سورة الكهف، لم يخشه، ولا يقدر أن يفتنه، وكانت له تلك الآية كالتميمة من الدجال، فطوبى لمن نجا بإيمانه قبل فتن الدجال وهوانه وصغاره، وليدركن أقواما مثل خيار أصحاب محمد صلى الله عليه وسلم
It was said: The dog of the hour is the Dajjal, and whoever is patient with the temptation of the Dajjal will not be tempted, and will never be tempted alive or dead, and whoever catches him and does not follow him will be granted Paradise, and if a man is saved and the Dajjal lies once, and says: I know who you are, you are the Dajjal, then he recites to him the opening of Surah Al-Kahf, he does not fear him, and he is not able to tempt him, and that verse was for him like an amulet from the Dajjal, so blessed is he who was saved by his faith before the temptations of the Dajjal. And his humiliation and his children, and they will reach people like the best companions of Muhammad, ﷺ
بقية بن الوليدصفوبن عمروشريح بن عبيدكعب الأحبارautomatic reading, unverified