Musannaf Ibn Abi Shayba 28:593

Muhammad ibn Fudayl narrated to us, from Yahya ibn Sa'id, who said: It was narrated to me that Abu Bakr sent armies to Al-Sham, and he went out following Yazid ibn Abi Sufyan. He said: "I advise you with ten [things]: Do not kill a child, nor a woman, nor an elderly person who is decrepit. Do not cut down a fruit-bearing tree, nor destroy a populated place, nor slaughter a sheep or a camel except for food. Do not drown a palm tree, nor burn it, and do not act treacherously, and do not be cowardly."