Sunan At-Tirmidhi

Narrated Abu Ishaq:hearing Al-Bara say: ""There was a man reciting [Surat] 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