Sayyiduna Khalid bin Zayd (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him), seeing off the people of Mu'tah, went as far as Thaniyyat al-Wada'. He (peace and blessings be upon him) stopped, and the people also stopped around him (peace and blessings be upon him). He (peace and blessings be upon him) said: "Go forth on campaign in the name of Allah. Fight your enemies and the enemies of Allah in the land of Syria. You will soon find people in churches, secluded from other people; do not molest them. And (you will find) another kind of people on whose heads devils have made nests; split those with the sword. Do not kill any woman, small child or very old person; do not cut down trees or palms; and do not demolish houses."
This too is munqati' (broken in its chain) and weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18156