Sahih Muslim Book - 6, Hadith - 1856

وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا أَبُو خَيْثَمَةَ عَنْ أَبِى 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 النَّبِىَّ -صلى الله عليه وسلم- فَذَكَرَ ذَلِكَ لَهُ فَقَالَ « تِلْكَ السَّكِينَةُ تَنَزَّلَتْ لِلْقُرْآنِ ».

It was narrated that Al-Bara' said: "A man was reciting Surat Al-Kahf, and he had a horse with him that was tied with two ropes. A cloud overshadowed it and started spinning and coming closer, and his horse was startled by it. The next morning he came to the Prophet (s.a.w) and told him about that. He said: 'That was tranquility (Sakinah) that came down because of the Qur'an."'