Musnad Ahmad 32:1012

Muhammad ibn Abdullah—meaning al-Ansari—narrated to us, Humayd narrated to us, from Anas, who said: "The Prophet, peace and blessings of Allah be upon him, heard the crying of a young boy while he was in prayer, so he lightened it. We thought that he only did that out of mercy for the boy, since he knew that his mother was with him in the prayer."