Sahih Bukhari Volume 4, Book 61, Hadith 3616

حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، عَنْ أَبِي إِسْحَاقَ، سَمِعْتُ الْبَرَاءَ بْنَ عَازِبٍ ـ رضى الله عنهما ـ قَرَأَ رَجُلٌ الْكَهْفَ وَفِي الدَّارِ الدَّابَّةُ فَجَعَلَتْ تَنْفِرُ فَسَلَّمَ، فَإِذَا ضَبَابَةٌ ـ أَوْ سَحَابَةٌ ـ غَشِيَتْهُ، فَذَكَرَهُ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ ‏"‏ اقْرَأْ فُلاَنُ، فَإِنَّهَا السَّكِينَةُ نَزَلَتْ لِلْقُرْآنِ، أَوْ تَنَزَّلَتْ لِلْقُرْآنِ ‏"

Narrated By Al-Bara' bin 'Azib : A man recited Surat-al-Kahf (in his prayer) and in the house there was a (riding) animal which got frightened and started jumping. The man finished his prayer with Taslim, but behold! A mist or a cloud hovered over him. He informed the Prophet of that and the Prophet said, "O so-and-so! Recite, for this (mist or cloud) was a sign of peace descending for the recitation of Qur'an."