That was tranquillity which came down at the recitation of the Qur'an.  

كَانَ رَجُلٌ يَقْرَأُ سُورَةَ الْكَهْفِ وَعِنْدَهُ فَرَسٌ مَرْبُوطٌ بِشَطَنَيْنِ فَتَغَشَّتْهُ سَحَابَةٌ فَجَعَلَتْ تَدُورُ وَتَدْنُو وَجَعَلَ فَرَسُهُ يَنْفِرُ مِنْهَا فَلَمَّا أَصْبَحَ أَتَى النَّبِيَّ ﷺ فَذَكَرَ ذَلِكَ لَهُ فَقَالَ تِلْكَ السَّكِينَةُ تَنَزَّلَتْ لِلْقُرْآنِ  

[Machine] There was a man reading Surah Al-Kahf in his house, and next to him was his horse tied to two stakes, until a cloud came and approached him. The cloud kept coming closer and closer until his horse became scared of it. The man said, "I was amazed by this." When morning came, he went to the Prophet ﷺ and mentioned this to him, narrating the story. The Prophet ﷺ said, "That was tranquility descending with the recitation of the Quran."  

كَانَ رَجُلٌ يَقْرَأُ فِي دَارِهِ سُورَةَ الْكَهْفِ وَإِلَى جَانِبِهِ حِصَانٌ لَهُ مَرْبُوطٌ بِشَطَنَيْنِ حَتَّى غَشِيَتْهُ سَحَابَةٌ فَجَعَلَتْ تَدْنُو وَتَدْنُو حَتَّى جَعَلَ فَرَسُهُ يَنْفِرُ مِنْهَا قَالَ الرَّجُلُ فَعَجِبْتُ لِذَلِكَ فَلَمَّا أَصْبَحَ أَتَى النَّبِيَّ ﷺ فَذَكَرَ ذَلِكَ لَهُ وَقَصَّ عَلَيْهِ فَقَالَ النَّبِيُّ ﷺ تِلْكَ السَّكِينَةُ تَنَزَّلَتْ لِلْقُرْآنِ  

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." (Using translation from Bukhārī 5011)   

كَانَ رَجُلٌ يَقْرَأُ فِي دَارِهِ سُورَةَ الْكَهْفِ وَإِلَى جَانِبِهِ حِصَانٌ مَرْبُوطٌ حَتَّى تَغَشَّتْهُ سَحَابَةٌ فَجَعَلَتْ تَدْنُو وَتَدْنُو حَتَّى جَعَلَ الْفَرَسُ يَفِرُّ مِنْهَا قَالَ الرَّجُلُ فَعَجِبْتُ لِذَلِكَ فَلَمَّا أَصْبَحَ أَتَى النَّبِيَّ ﷺ فَذَكَرَ لَهُ وَقَصَّ عَلَيْهِ فَقَالَ النَّبِيُّ ﷺ «تِلْكَ السَّكِينَةُ تَنَزَّلَتْ لِلْقُرْآنِ»  

"تِلْكَ السَّكِينَةُ تَنَزَّلَتْ بالْقُرْآنِ" وذلك أَن رجلا قال: يا رسول اللَّه: كنْتُ أَقْرَأُ سُورةَ الْكَهْفِ، وَإلَى جانبى حصانٌ مربوطٌ بِشَطنَيْنِ، فَتَعشَّتْنى سحابةٌ، فجعلت تَدْنُو وَتَدْنُو، وجعل هذا الفرس يَنْفِرُ، فقال ذلِكَ".