A man was reciting Surah al-Kahf, and beside him was a horse tied with two ropes. A cloud came over him and began to draw nearer and nearer, and his horse started to shy away. In the morning, he came to the Prophet ﷺ and mentioned this to him. The Prophet ﷺ said, "That was tranquility, which descended on account of the Quran."
كان رجل يقرأ سورة الكهف، وإلى جانبه حصان مربوط بشطنين، فتغشته سحابة، فجعلت تدنو، وتدنو، وجعل فرسه ينفر، فلما أصبح أتى النبي ﷺ، فذكر ذلك له، فقال: تلك السكينة، تنزلت بالقرآن.
Bukhari related it in the Excellences of the Quran (5011), and Muslim in the Prayer of Travelers (795:240), both through the route of Abu Khaythama Zuhayr bin Harb, from Abu Ishaq, from Bara bin Azib, who said, and then he mentioned it.His saying "shatan" means the long rope.