It is narrated from Sayyiduna ‘Isam that whenever he dispatched a detachment of the army, he would instruct them that if you see a mosque in that area or hear the voice of a muezzin, then do not kill anyone there.
Hadith Referenceمسند احمد / مسند المكيين / 15714
Hadith Gradingحکم دارالسلام:إسناده ضعيف لجهالة ابن عصام المزني، وعبدالملك بن نوفل