Musannaf Ibn Abi Shayba 28:608

Affan narrated to us, he said: Abd al-Wahid ibn Ziyad narrated to us, he said: Abu Rawq Atiyyah ibn al-Harith narrated to us, he said: Abu al-Ghurayf Ubayd Allah ibn Khalifah narrated to us, from Safwan ibn Assal, that the Prophet, may Allah bless him and grant him peace, when he would send out a military expedition, he would say: "Do not kill a newborn child."