ذَكَرُوا النَّارَ وَالنَّاقُوسَ، فَذَكَرُوا الْيَهُودَ وَالنَّصَارَى، فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ.
Anas ibn Malik reported: They mentioned fire and the bell (naqus), and they mentioned the Jews and the Christians. Then Bilal was commanded to say the adhan in pairs and the iqamah singly.
also in Sahih al-Bukhari 605, Sahih al-Bukhari 606, Sahih al-Bukhari 607