Musnad Ahmad 658:124

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Abu Ishaq, from al-Bara’ ibn ‘Azib, who said: A man was reciting Surat al-Kahf in his house, and beside him was his horse tied with two ropes. A cloud covered him, and it began to draw nearer and nearer, until his horse began to shy away from it. The man said: I was amazed by that. When morning came, he went to the Prophet, peace and blessings of Allah be upon him, and mentioned that to him and narrated it to him. The Prophet, peace and blessings of Allah be upon him, said: "That is the tranquility (al-sakinah) that descended for the Quran."