While one of the companions of the Prophet ﷺ was praying, his stallion, which was tied up in the courtyard, began to startle and bolt. The man went out to check but saw nothing, and the horse continued to startle. In the morning he mentioned this to the Prophet ﷺ, who said, "That was tranquility descending with the Quran."
حدثنا حجين حدثنا إسرائيل عن أبي إسحاق
بينما رجل من أصحاب النبي ﷺ يصلي وفرس له حصان مربوط في الدار فجعل ينفر فخرج الرجل فنظر فلم ير شيئا وجعل ينفر فلما أصبح ذكر ذلك للنبي ﷺ فقال تلك السكينة نزلت بالقرآن
Its isnad is sahih. Hujayn is Ibn Muthanna, and he is a well-known trustworthy narrator. The hadith has previously appeared at 18499.