Sahih Muslim Book - 6, Hadith - 1857

وَحَدَّثَنَا ابْنُ الْمُثَنَّى وَابْنُ بَشَّارٍ - وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ حَدَّثَنَا شُعْبَةُ عَنْ أَبِى إِسْحَاقَ قَالَ سَمِعْتُ الْبَرَاءَ يَقُولُ قَرَأَ رَجُلٌ الْكَهْفَ وَفِى الدَّارِ دَابَّةٌ فَجَعَلَتْ تَنْفِرُ فَنَظَرَ فَإِذَا ضَبَابَةٌ أَوْ سَحَابَةٌ قَدْ غَشِيَتْهُ قَالَ فَذَكَرَ ذَلِكَ لِلنَّبِىِّ -صلى الله عليه وسلم- فَقَالَ « اقْرَأْ فُلاَنُ فَإِنَّهَا السَّكِينَةُ تَنَزَّلَتْ عِنْدَ الْقُرْآنِ أَوْ تَنَزَّلَتْ لِلْقُرْآنِ ».

It was narrated that Abu Ishaq said: "I heard Al-Bara' say: 'A man recited Surat Al-Kahf, and in the house there was an animal which became startled. He looked and saw that a cloud had overshadowed it. He mentioned that to the Prophet (s.a.w) and he said: Recite, so-and-so, for it is tranquility (Sakinah) that came down when the Qur'an (is recited)" or "that came down because of the Qur'an.'"