Musnad Ahmad 32:1579

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Sulayman ibn Bilal narrated to us, Sharik—meaning Ibn Abi Namir—narrated to us, from Anas ibn Malik, who said: "I have never prayed behind an Imam who was lighter in prayer than the Messenger of Allah, may Allah bless him and grant him peace, nor more complete. And if the Messenger of Allah, may Allah bless him and grant him peace, were to hear the crying of a child behind him, he would lighten the prayer, fearing that it might be difficult for his mother."