A man was reciting Surah al-Kahf when an animal in the house became startled and started shying away. He looked up and saw a mist or cloud overshadowing him. He mentioned this to the Prophet ﷺ, who said, "Keep reciting, so-and-so. That was tranquility descending at the recitation of the Quran," or "descending for the Quran."
قرأ رجل الكهف وفي الدار دابة، فجعلت تنفر، فنظر فإذا ضبابة أو سحابة قد غشيته، قال: فذكر ذلك للنبي ﷺ، فقال: اقرأ فلان، فإنها السكينة تنزلت عند القرآن أو تنزلت للقرآن.
Bukhari narrated it in Manaqib (3614) and Muslim in Prayer of the Traveler (795:241), both 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 he related it. The wording is Muslim's. Some versions of the hadith mention that this man was Usayd bin Hudayr.