Sahih Bukhari Volume 5, Book 64, Hadith 4234

قَالَ أَبُو بُرْدَةَ عَنْ أَبِي مُوسَى، قَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ إِنِّي لأَعْرِفُ أَصْوَاتَ رُفْقَةِ الأَشْعَرِيِّينَ بِالْقُرْآنِ، حِينَ يَدْخُلُونَ بِاللَّيْلِ، وَأَعْرِفُ مَنَازِلَهُمْ مِنْ أَصْوَاتِهِمْ بِالْقُرْآنِ بِاللَّيْلِ، وَإِنْ كُنْتُ لَمْ أَرَ مَنَازِلَهُمْ حِينَ نَزَلُوا بِالنَّهَارِ، وَمِنْهُمْ حَكِيمٌ، إِذَا لَقِيَ الْخَيْلَ ـ أَوْ قَالَ الْعَدُوَّ ـ قَالَ لَهُمْ إِنَّ أَصْحَابِي يَأْمُرُونَكُمْ أَنْ تَنْظُرُوهُمْ ‏"‏‏

Narrated Abu Burda: Abu Musa said, "The Prophet said, "I recognize the voice of the group of Al-Ashariyun, when they recite the Qur'an, when they enter their homes at night, and I recognize their houses by (listening) to their voices when they are reciting the Qur'an at night although I have not seen their houses when they came to them during the day time. Amongst them is Hakim who, on meeting the cavalry or the enemy, used to say to them (i.e. the enemy). My companions order you to wait for them.'"