Full Sharh — Riyad as-Salihin 319
وعن البراء بن عازب رضي الله عنهما قال: كان رجل يقرأ سورة الكهف، وعنده فرس مربوط بشطنين فتغشته سحابة فجعلت تدنو، وجعل فرسه ينفر منها. فلما أصبح أتى النبي صلى الله عليه وسلم ، فذكر له ذلك فقال : " تلك السكينة تنزلت للقرآن" ((متفق عليه)) .
English Translation
Al-Bara' bin 'Azib (May Allah be pleased with them) reported:: A man was reciting Surat Al-Kahf, and a horse was tied with two ropes beside him. As he was reciting, a cloud overshadowed him, and as it began to come nearer and nearer, the horse began to trample voilently. The man came to the Messenger of Allah (ﷺ) in the morning and mentioned the incident to him. He (ﷺ) said, "That was tranquillity which descended as a result of the recitation of the Qur'an." .
Source
Riyad as-Salihin(رياض الصالحين)
The Book of Virtues
Hadith Number
319