Maliks Muwatta Book 21, Hadith Number 10.
Yahya related to me from Malik from Yahya ibn Said that Abu Bakr as-Siddiq was sending armies to ash-Sham. He went for a walk with Yazid ibn Abi Sufyan who was the commander of one of the battalions. It is claimed that Yazid said to Abu Bakr, “Will you ride or shall I get down?” Abu Bakrsaid, “I will not ride and you will not get down. I intend these steps of mine to be in the way of Allah.”
Then Abu Bakr advised Yazid, “You will find a people who claim to have totally given themselves to Allah. Leave them to what they claim to have given themselves. You will find a people who have shaved the middle of their heads, strike what they have shaved with the sword.
“I advise you ten things| Do not kill women or children or an aged, infirm person. Do not cut down fruit-bearing trees. Do not destroy an inhabited place. Do not slaughter sheep or camels except for food. Do not burn bees and do not scatter them. Do not steal from the booty, and do not be cowardly.”