Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
Bukhari :: Book 6 :: Volume 60 :: Hadith 363
Narrated Al-Bara:
While a man from the companions of the Prophet was reciting (Quran) and his horse was tied in the house, the horse got startled and started jumping. The man came out, looked around but could not find anything, yet the horse went on jumping. The next morning he mentioned that to the Prophet.
The Prophet said, "That was the tranquility (calmness) which descended because of the recitation of the Quran."