Narrated Anas :
The people mentioned the fire and the bell. They suggested those as signals to indicate the starting of prayers). By that they mentioned the Jews and the Christians.
Then Bilal was ordered to pronounce Adhan for the prayer by saying its wordings twice. 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). (Sahih al- Bukhari 603)