A man was reciting Surah al-Kahf while there was an animal in the house, and it began to shy away. He looked up and saw a mist or a cloud that had covered him. He mentioned this to the Prophet ﷺ, who said, "Keep reciting, so-and-so, for that was tranquility descending with 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 the Book of Virtues (3614), and Muslim in the Book of the Prayer of Travelers (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 saying, then he mentioned it. Some versions of the hadith state that this man was Usayd bin Hudayr.