Musnad Ahmad 32:1757

'Affan narrated to us, Hammad bin Zayd narrated to us, he said: 'Ali bin Zayd and Humayd informed us, from Anas bin Malik, that the Messenger of Allah (may Allah bless him and grant him peace) shortened [the prayer] one day during the dawn prayer. It was said: "O Messenger of Allah, why did you shorten it?" He said: "I heard the crying of a child, and I thought that his mother was with us praying, so I wanted to free his mother for him." Hammad also said: "I thought that his mother was praying with us, so I wanted to free his mother for him." 'Affan said: "I found it with me in more than one place, from 'Ali bin Zayd, Humayd, and Thabit, from Anas bin Malik."