Musnad Ahmad 658:42

'Affan narrated to us, Shu'bah narrated to us, from Abu Ishaq, who said: I heard Al-Bara' say: A man recited Surat Al-Kahf while he had a beast tied up, and the beast began to shy away. The man looked at a cloud that had covered him, or a mist, and he became frightened. He went to the Prophet, peace and blessings of Allah be upon him. I said: Did the Prophet, peace and blessings of Allah be upon him, name that man? He said: Yes. He said: "Read, so-and-so, for the tranquility descended for the Quran, or at the Quran."