Sahih al-Bukhari 603
Narrated Anas:
The people mentioned the fire and the bell (they suggested those as signals to indicate the starting of 
prayers), and by that they mentioned the Jews and the Christians. Then Bilal was ordered to 
pronounce Adhan for the prayer by saying its wordings twice, and for the Iqama (the call for the 
actual standing for the prayers in rows) by saying its wordings once. (Iqama is pronounced when the 
people are ready for the prayer).
حَدَّثَنَا عِمْرَانُ بْنُ مَيْسَرَةَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا خَالِدٌ الْحَذَّاءُ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ ذَكَرُوا النَّارَ وَالنَّاقُوسَ، فَذَكَرُوا الْيَهُودَ وَالنَّصَارَى، فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ‏.‏