Musannaf Ibn Abi Shayba 28:594

Jarir ibn Abd al-Hamid narrated to us, from Layth, from Mujahid, who said: In war, a child is not to be killed, nor a woman, nor an elderly person who is frail. Food is not to be burned, nor palm trees. Houses are not to be destroyed, and fruit-bearing trees are not to be cut down.