Musannaf Ibn Abi Shayba 28:586

Isma'il ibn 'Ulayyah narrated to us, from Ayyub, who said: I heard a man narrating from my father, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent out a military expedition (sariyyah) in which I was. He said: He forbade us from killing the hired laborers (al-'usafa') and the young servants (al-wasafa').