Musnad Ahmad 132:18

Hassan narrated to us, Ibn Lahi'ah narrated to us, Zabban narrated to us, from Sahl ibn Mu'adh, from his father, from the Messenger of Allah, peace and blessings of Allah be upon him, that he said: "Whoever reads the beginning of Surat al-Kahf and its end, it will be a light for him from his feet to his head. And whoever reads it entirely, it will be a light for him between the heaven and the earth."