The Prophet ﷺ heard the cry of a child during the prayer, so he shortened the prayer.
حدثنا يحيى عن ابن عجلان قال سمعت أبي
سمع النبي ﷺ صوت صبي في الصلاة فخفف الصلاة
Its chain is sahih. Bukhari related it, 2/202, no. 707, in the chapter on adhan, on shortening the prayer when a child cries, and Muslim, 1/343, no. 470, in the chapter on prayer, on commanding imams to shorten the prayer.