Sahih Al Bukhari (475)

عن أنس بن مالك قال "ذكروا النار والناقوس فذكروا اليهود والنصارى فأمر بلال أن يشفع الأذان وأن يوتر الإقامة"

Anas (may Allah be pleased with him) narrated: 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 Iqamah (the call for the actual standing for the prayers in rows) by saying its wordings once. (Iqamah is pronounced when the people are ready for the prayer).