A man was reciting Surah Al-Kahf and his horse was tied with two ropes beside him. A cloud came down and spread over that man and it kept 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 ﷺ and told him of that experience. the Prophet ﷺ says, "That was al-sakiinah (tranquillity) which descended because of (the recitation) of the Qur'aan. Ibn Hajar al-'Asqalani explains that al-sakiinah referred to here are the angels because wherever angels go, peace and tranquillity descend. Such is the beauty of Surah al-Kahf that it is this peace and tranquillity that descended upon its recitation. Reference for hadith: Sahih Al Bukhari. This passage excerpt is taken from Yasir Qadhi's book: Lessons from Surah Al-Kahf.