Sahih al-Bukhari Book 10, Hadith 2
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 Messenger (ﷺ) ordered Bilal to get up
and pronounce the Adhan for prayers.
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).
Bilal was ordered to repeat the wording of the Adhan for prayers twice, and to pronounce the wording of the Iqama once except "Qad-qamat-is-salat".