Call to Prayers (Adhaan)

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).
1.578:
Narrated Ibn `Umar: When the Muslims arrived at Medina, they
used to assemble for the prayer, and used to guess the time
for it. During those days, the practice of Adhan for the
prayers had not been introduced yet. Once they discussed this
problem regarding the call for prayer. Some people suggested
the use of a bell like the Christians, others proposed a
trumpet like the horn used by the Jews, but `Umar was the
first to suggest that a man should call (the people) for the
prayer; so Allah's Apostle ordered Bilal to get up and
pronounce the Adhan for prayers. Bukhari 1/577.