Mishkat al-Masabih 3935
Islam al-Muzani said:
God’s Messenger sent us in a detachment and said, “When you see a mosque or hear a mu’adhdhin (The one who makes the call to prayer) do not kill anyone.” 

Tirmidhi and Abu Dawud transmitted it.
وَعَن عصامٍ المزنيِّ قَالَ بَعَثَنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي سَرِيَّةٍ فَقَالَ: «إِذَا رَأَيْتُمْ مَسْجِدًا أَوْ سَمِعْتُمْ مُؤَذِّنًا فَلَا تَقْتُلُوا أَحَدًا» . رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد