Musannaf Ibn Abi Shayba 28:395

Waki' narrated to us, he said: Sufyan narrated to us, from Alqamah ibn Marthad, from Sulayman ibn Buraydah, from his father, who said: When the Messenger of Allah (may Allah bless him and grant him peace) would send a commander over a detachment or an army, he would advise him regarding his own self to fear Allah, and regarding those who were with him from the Muslims to treat them well. He would say: "Fight in the way of Allah. Fight those who disbelieve in Allah. Fight, but do not commit treachery, do not break your pledges, do not mutilate, and do not kill a newborn child."