Sahih Bukhari Volume 6, Book 61, Hadith Number 531.

Narrated By Al-Bara’ : A man was reciting Surat Al-Kahf and his horse was tied with two ropes beside him. A cloud came down and spread over that man, and it kept on 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, and told him of that experience. The Prophet said, “That was As-Sakina (tranquility) which descended because of (the recitation of) the Qur’an.”