The Prophet ﷺ heard the cry of a child during the prayer, so he shortened it.
سمع النبي ﷺ صوت صبي في الصلاة، فخفف الصلاة.
Imam Ahmad (9581) narrated it from Yahya (that is, Qattan), from Ibn Ajlan, who said, "I heard my father, from Abu Hurairah," who said, and then he related it. Its chain is hasan because of Ibn Ajlan and his father, for the two of them are hasan in narration.Haythami said in Majma (2/90), "Ahmad narrated it, and its narrators are the narrators of Sahih."