That was tranquillity which came down at the recitation of the Quran.
Sahih Muslim Book 6, Hadith 284
← Previous Hadith (The Holy Prophet) was reciting Surat al-Fath as he was travelling on his mount.

Sahih Muslim Book 6, Hadith 283 Next Hadith → I heard al-Bara' as saying that a man recited al-Kahf when an animal was there in the house and it began to take fright. And as he looked around, he found a cloud overshadowing it. He mentioned that to the Messenger of Allah (ﷺ). Upon this he said: O so and so, recite on (the surah) as- Sakina descends at the (recitation of the Quran) or on account (of the recitation) of the Quran.

Sahih Muslim Book 6, Hadith 285