Abu Imran al-Jawni (may Allah have mercy on him) says that Abu Bakr (may Allah be pleased with him) sent Yazid bin Abi Sufyan (may Allah be pleased with him) towards Syria and walked with him on foot. Yazid said: I do not like that you should be on foot while I am riding. Abu Bakr (may Allah be pleased with him) said: You are going out to fight in the path of Allah, and I intend reward in walking on foot. Then he advised him: Do not kill a child, a woman, a very old man, a sick person or a monk; do not cut down fruit-bearing trees; do not lay waste inhabited land; slaughter cows and camels only for food; and do not drown palm trees nor burn them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18152