A man was reciting Surah al-Kahf, and in the courtyard there was an animal that began to startle and shy away. When he finished his recitation with the salam, he noticed a mist or cloud had covered him. He mentioned this to the Prophet ﷺ, who said, "Recite, so-and-so, for that was tranquility descending for the Quran," or, "coming 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 the Prayer of Travelers (795), both from the hadith of Shubah, from Abu Ishaq, who said, "I heard Bara bin Azib," and then he related it. Both of them also narrated it from Zuhayr Abu Khaythama, from Abu Ishaq with his chain, and in it, "There was a man reciting Surah Kahf, and beside him was a horse tied with two ropes. A cloud came over him and kept drawing nearer and nearer, and his horse began to shy away. When morning came, he went to the Prophetﷺ and mentioned this to him, and he said, "That was the sakinah descending because of the Quran."Bukhari narrated it in the Merits of the Quran (5011) from Amr bin Khalid, and Muslim from Yahya bin Yahya, both of them from Zuhayr Abu Khaythama.As for the sakinah, it has been said that it refers to the angels, and it has also been said that it refers to that which brings about tranquility and clarity of heart. As for the shatan, it is the long rope.