Musnad Ahmad 32:644

Ibrahim ibn Mahdi narrated to us, Ja'far ibn Sulayman narrated to us, he said: Thabit al-Bunani narrated to us, from Anas ibn Malik, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, would hear the crying of a child with his mother while he was in prayer, so he would recite a light surah." Ja'far said: "Or a short surah."