Language:
كتاب فضائل القرآن
66  
Virtues of the Qur'an
(11)
Chapter: The superiority of Surat Al-Kahf (No.18)
(11)
باب فَضْلُ الْكَهْفِ
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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 5011
In-book reference : Book 66, Hadith 33
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 531
  (deprecated numbering scheme)