Ibn Isam narrates from his father (may Allah be pleased with him) that whenever the Prophet (peace and blessings be upon him) sent out a small detachment, he would say: "When you hear a mu'adhdhin (caller to prayer) or see a mosque, do not kill anyone."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18625