Yahya bin Sa'id (may Allah have mercy on him) says that Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) dispatched armies towards Syria, and Sayyiduna Abu Bakr (may Allah be pleased with him) walked on foot with Sayyiduna Yazid bin Abi Sufyan (may Allah be pleased with him), who was the commander of one quarter of the army. People say that Sayyiduna Yazid (may Allah be pleased with him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): Either you ride, or else I will walk too. Sayyiduna Abu Bakr (may Allah be pleased with him) said: "You will not dismount, nor will I ride; I intend reward in the path of Allah through these steps." Then he said: "You will soon find people who claim that they have devoted themselves to Allah; leave them alone, and leave alone that for which they claim to have set themselves apart. And you will find a people who have made nests of hair on their heads; cut those (parts) with the sword. And I give you ten instructions: Do not kill any woman, child or old man; do not cut down any fruit-bearing tree; do not lay waste and desolate any inhabited land; slaughter a goat or a camel only for food; do not burn palm trees nor drown them; and do not steal from the spoils nor show cowardice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18148