Musannaf Ibn Abi Shayba 28:592

Muhammad ibn Fudayl narrated to us, from Yazid ibn Abi Ziyad, from Zayd ibn Wahb, who said: The letter of Umar came to us: "Do not commit ghulul (stealing from the spoils of war), do not commit treachery, and do not kill a newborn child. And fear Allah regarding the farmers."