Al-Bara' reported that a person was reciting Surat al-Kahf and there was a horse tied with two ropes at his side, a cloud overshadowed him, and as it began to come nearer and nearer his horse began to take fright from it. He went and mentioned that to the Prophet (ﷺ) in the morning, and he (the Holy Prophet) said: That was tranquillity which came down at the recitation of the Qur'an.
Source: sunnah.com
Other translations
Arabic
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْبَرَاءِ، قَالَ كَانَ رَجُلٌ يَقْرَأُ سُورَةَ الْكَهْفِ وَعِنْدَهُ فَرَسٌ مَرْبُوطٌ بِشَطَنَيْنِ فَتَغَشَّتْهُ سَحَابَةٌ فَجَعَلَتْ تَدُورُ وَتَدْنُو وَجَعَلَ فَرَسُهُ يَنْفِرُ مِنْهَا فَلَمَّا أَصْبَحَ أَتَى النَّبِيَّ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ فَقَالَ " تِلْكَ السَّكِينَةُ تَنَزَّلَتْ لِلْقُرْآنِ " .
sunnah.com