From Sahih Muslim (Hadith 1856):
Al-Bara' reported that a person was reciting Surat al-Kahf and there was a horse tied with two ropes at his side, a cloud overshadowed him, and as it began to come nearer and nearer his horse began to take fright from it. He went and mentioned that to the Prophet (ﷺ) in the morning, and he (the Holy Prophet) said:That was tranquillity which came down at the recitation of the Qur'an
Reference: Muslim 1856