(1) باب بَدْءُ الأَذَانِ
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).
Reference: Sahih al-Bukhari 603
In-book reference: Book 10, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 577