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).