Musnad Ahmad 32:1501

Ubayd ibn Abi Qurrah narrated to us, Sulayman ibn Bilal narrated to us, from Sharik, that he heard Anas ibn Malik saying: "I have not 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, heard the crying of a child, he would lighten the prayer, fearing that his mother might be distressed."