Musnad Abu Ya'la al-Mawsili 117:102

Ubayd Allah ibn Umar al-Jushami narrated to us, Ja'far narrated to us, from Thabit, from Anas, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) heard the crying of a child with his mother during the prayer, he would recite a light or short surah. Ja'far was in doubt.