Musnad Ahmad 32:1189

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, who said: "While the Messenger of Allah (peace and blessings of Allah be upon him) was praying, he heard the crying of a child, so he shortened his prayer. We thought that he only lightened it because of the child, [and] that his mother was in the prayer."