"He it is Who sent down As-Sakīnah (tranquillity and calmness) into the hearts of the believers..." (V.48:4) Sahih al-Bukhari 4839
Arabic hadith text
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، عَنْ إِسْرَائِيلَ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْبَرَاءِ ـ رضى الله عنه ـ قَالَ بَيْنَمَا رَجُلٌ مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم يَقْرَأُ، وَفَرَسٌ لَهُ مَرْبُوطٌ فِي الدَّارِ، فَجَعَلَ يَنْفِرُ، فَخَرَجَ الرَّجُلُ فَنَظَرَ فَلَمْ يَرَ شَيْئًا، وَجَعَلَ يَنْفِرُ، فَلَمَّا أَصْبَحَ ذَكَرَ ذَلِكَ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ " تِلْكَالسَّكِينَةُ تَنَزَّلَتْ بِالْقُرْآنِ "
Translation
Narrated Al-Bara
While a man from the companions of the Prophet (ﷺ) was reciting (Qur'an) 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 (ﷺ) said, "That was the tranquility (calmness) which descended because of the recitation of the Qur'an."