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  

I heard al-Bara' as saying that a man recited al-Kahf when an animal was there in the house and it began to take fright. And as he looked around, he found a cloud overshadowing it. He mentioned that to the Messenger of Allah ﷺ. Upon this he said: O so and so, recite on (the surah) as- Sakina descends at the (recitation of the Qur'an) or on account (of the recitation) of the Qur'an.  

سَمِعْتُ الْبَرَاءَ يَقُولُ قَرَأَ رَجُلٌ الْكَهْفَ وَفِي الدَّارِ دَابَّةٌ فَجَعَلَتْ تَنْفِرُ فَنَظَرَ فَإِذَا ضَبَابَةٌ أَوْ سَحَابَةٌ قَدْ غَشِيَتْهُ قَالَ فَذَكَرَ ذَلِكَ لِلنَّبِيِّ ﷺ فَقَالَ اقْرَأْ فُلاَنُ فَإِنَّهَا السَّكِينَةُ تَنَزَّلَتْ عِنْدَ الْقُرْآنِ أَوْ تَنَزَّلَتْ لِلْقُرْآنِ  

This hadith has been narrated on the authority of al-Bara' with a slight modification of words.  

سَمِعْتُ الْبَرَاءَ يَقُولُ فَذَكَرَا نَحْوَهُ غَيْرَ أَنَّهُمَا قَالاَ تَنْقُزُ  

I was afraid lest it should trample (his son) Yahya. I stood near it (the horse) and saw something like a canopy over my head with what seemed to be lamps in it, rising up in the sky till it disappeared. I went to the Messenger of Allah ﷺ on the next day and said: Messenger of Allah, I recited the Qur'an during the night in my enclosure and my horse began to jump. Upon this the Messenger of Allah ﷺ said: You should have kept on reciting, Ibn Hudair. He (Ibn Hudair) said: I recited. It jumped (as before). Upon this the Messenger of Allah ﷺ again said: You should have kept on reciting, Ibn Hudair. He (Ibn Hudair) said: I recited and it again jumped (as before). The Messenger of Allah ﷺ again said: You should kave kept on reciting, Ibu Hudair. He (Ibn Hudair) said: (Messenger of Allah) I finished (the recitation) for Yahya was near (the horse) and I was afraid lest it should trample him. I saw something like a canopy with what seemed to be lamps in it rising up in the sky till it disappeared. Upon this the Messenger of Allah ﷺ said: Those were the angels who listened to you; and if you had continued reciting, the people would have seen them in the morning and they would not have concealed themselves from them.  

أَنْ تَطَأَ يَحْيَى فَقُمْتُ إِلَيْهَا فَإِذَا مِثْلُ الظُّلَّةِ فَوْقَ رَأْسِي فِيهَا أَمْثَالُ السُّرُجِ عَرَجَتْ فِي الْجَوِّ حَتَّى مَا أَرَاهَا قَالَ فَغَدَوْتُ عَلَى رَسُولِ اللَّهِ ﷺ فَقُلْتُ يَا رَسُولَ اللَّهِ بَيْنَمَا أَنَا الْبَارِحَةَ مِنْ جَوْفِ اللَّيْلِ أَقْرَأُ فِي مِرْبَدِي إِذْ جَالَتْ فَرَسِي فَقَالَ رَسُولُ اللَّهِ ﷺ اقْرَإِ ابْنَ حُضَيْرٍ قَالَ فَقَرَأْتُ ثُمَّ جَالَتْ أَيْضًا فَقَالَ رَسُولُ اللَّهِ ﷺ اقْرَإِ ابْنَ حُضَيْرٍ قَالَ فَقَرَأْتُ ثُمَّ جَالَتْ أَيْضًا فَقَالَ رَسُولُ اللَّهِ ﷺ اقْرَإِ ابْنَ حُضَيْرٍ قَالَ فَانْصَرَفْتُ وَكَانَ يَحْيَى قَرِيبًا مِنْهَا خَشِيتُ أَنْ تَطَأَهُ فَرَأَيْتُ مِثْلَ الظُّلَّةِ فِيهَا أَمْثَالُ السُّرُجِ عَرَجَتْ فِي الْجَوِّ حَتَّى مَا أَرَاهَا فَقَالَ رَسُولُ اللَّهِ ﷺ تِلْكَ الْمَلاَئِكَةُ كَانَتْ تَسْتَمِعُ لَكَ وَلَوْ قَرَأْتَ لأَصْبَحَتْ يَرَاهَا النَّاسُ مَا تَسْتَتِرُ مِنْهُمْ