A man was reciting Surah al-Kahf, and in the courtyard there was a riding animal that began to startle and shy away. Then he noticed a mist or cloud covering him. He mentioned this to the Prophet ﷺ, who said, "Recite, so-and-so, for that was the tranquility that descended for the Quran," or, "that came down for the Quran."
قرأ رجل الكهف وفي الدار الدابة، فجعلت تنفر فسلم، فإذا ضبابة أو سحابة غشيته، فذكره للنبي ﷺ فقال: اقرأ فلان، فإنها السكينة نزلت للقرآن أو تنزلت للقرآن.
Bukhari narrated it in the Book of Virtues (3614), and Muslim in the Book of Travelers' Prayer (795-241), both of them from Muhammad bin Bashshar, who said, Ghundar Muhammad bin Jafar narrated to us, who said, Shubah narrated to us, from Abu Ishaq, who said, I heard Bara say, and then he mentioned it. The wording given is Bukhari's.