Musnad Abu Ya'la al-Mawsili 129:7

Zuhayr ibn Harb narrated to us, Yahya ibn Sa'id narrated to us, Humayd narrated to us, from Anas ibn Malik, that the Prophet (may Allah bless him and grant him peace) heard the sound of a young boy while he was in prayer, so he lightened the prayer. And we thought that he lightened it because his mother was in the prayer.