Sahih al-Bukhari 5011
Narrated Al-Bara':
A man was reciting Surat Al-Kahf and his horse was tied with two ropes beside him. A cloud came 
down and spread over that man, and it kept on coming closer and closer to him till his horse started 
jumping (as if afraid of something). When it was morning, the man came to the Prophet, and told him 
of that experience. The Prophet (ﷺ) said, "That was As-Sakina (tranquility) which descended because of 
(the recitation of) the Qur'an."
حَدَّثَنَا عَمْرُو بْنُ خَالِدٍ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا أَبُو إِسْحَاقَ، عَنِ الْبَرَاءِ، قَالَ كَانَ رَجُلٌ يَقْرَأُ سُورَةَ الْكَهْفِ وَإِلَى جَانِبِهِ حِصَانٌ مَرْبُوطٌ بِشَطَنَيْنِ فَتَغَشَّتْهُ سَحَابَةٌ فَجَعَلَتْ تَدْنُو وَتَدْنُو وَجَعَلَ فَرَسُهُ يَنْفِرُ فَلَمَّا أَصْبَحَ أَتَى النَّبِيَّ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ فَقَالَ ‏
"‏ تِلْكَ السَّكِينَةُ تَنَزَّلَتْ بِالْقُرْآنِ ‏"‏‏.‏