Bilal was commanded (by the Messenger of Allah) to repeat (the phrases of) Adhan twice and once in Iqama. The narrator said: I made a mention of it before Ayyub who said: Except for saying: Qamat-is-Salat .
Sahih Muslim Book 4, Hadith 2
← Previous Hadith When the Muslims came to Medina, they gathered and sought to know the time of prayer but no one summoned them. One day they discussed the matter, and some of them said: Use something like the bell of the Christians and some of them said: Use horn like that of the Jews. Umar said: Why may not a be appointed who should call (people) to prayer? The Messenger of Allah (ﷺ) said: O Bilal, get up and summon (the people) to prayer.

Sahih Muslim Book 4, Hadith 1 Next Hadith → They (the Companions) discussed that they should know the timings of prayer by means of something recognized by all. Some of them said that fire should be lighted or a bell should be rung. But Bilal was ordered to repeat the phrases twice in Adhan, and once in Iqama.

Sahih Muslim Book 4, Hadith 3