Musannaf Ibn Abi Shayba 3:2302

Waki’ narrated to us, from Sufyan, from Abu al-Huwayrith al-Zurqi, from Ali ibn Qays, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Verily, I hear the crying of the child behind me, so I lighten [the prayer] out of compassion, lest I cause his mother to be distressed."