Musannaf Ibn Abi Shayba 3:2304

Narrated to us Sharik, from Abu Harun, from Abu Sa’id—as far as we know—from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, I am in the prayer and I hear the crying of a child, so I shorten it, fearing that I might burden his mother," or he said, "that his mother might be distressed."