Hadith #2884 (Jami' al-Tirmidhi)
Book: Chapters on The Virtues of the Qur'an
Narrator(s): Abdul Salam bin Harb bin Salm al-Mala'i عبد السلام بن حرب بن سلم النهدي الملائي, Ishaq bin Mansur al-Kausaj إسحاق بن منصور بن بهرام - الكوسج, Umar bin Ishaq al-Madni عمر بن إسحاق المدني مولى زائدة حجازي

hearing Al-Bara say: “There was a man reciting  Al-Kahf when he saw his riding animal stamping his feet, so he looked and there was something like a shadow or cloud, so he went to the Messenger of Allah (ﷺ) and mentioned that to him. The Prophet (ﷺ) said: “That was the tranquility which descends with the Qur’an, or descends because of the Qur’an.”

Source: https://parallelquran.com/hadith/jami-al-tirmidhi-2884/