Musannaf Ibn Abi Shayba 28:549

Ibn ‘Uyaynah narrated to us, from ‘Abd al-Malik ibn Nawfal ibn Musahiq, from a man from Muzaynah, from his father, who said: When the Prophet, peace and blessings of Allah be upon him, would send out a military expedition, he would say to them: "If you see a mosque or hear a caller to prayer, do not kill anyone."