It is narrated from Sayyiduna Mu'adh bin Anas al-Juhani (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever recites the beginning of Surah al-Kahf and its end, it will be a light for him from his feet to his head. And whoever recites all of it, it will be a light for him from his feet to his head, from the earth to the sky."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17577