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".
Sahih al-Bukhari Book 10, Hadith 3
← Previous Hadith 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.

Sahih al-Bukhari Book 10, Hadith 2 Next Hadith → When the number of Muslims increased they discussed the question as to how to know the time for the prayer by some familiar means. Some suggested that a fire be lit (at the time of the prayer) and others put forward the proposal to ring the bell. Bilal was ordered to pronounce the wording of Adhan twice and of the Iqama once only.

Sahih al-Bukhari Book 10, Hadith 4