When the Muslims arrived at Al-Madîna, they used to assemble for As-Salat (the prayer), and used to guess the time for it. During those days, the practice of Adhân 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 As-Salât (the prayer); so Allâh's Messenger (ﷺ) ordered Bilâl to get up and pronounce the Adhân for As-Salât (the prayer). [1:578-0.B.]

Bilâl was ordered to repeat the wording of the Adhân for Salât (prayers) twice (in doubles), and to pronounce the wording of the Iqama once (in singles) except Qad-qâmat-is-Salât. [1:579-O.B.]

Allâh's Messenger (ﷺ) said, "When the Adhân is pronounced Satan takes to his heels and passes wind with noise during his flight in order not to hear the Adhân. When the Adhân is completed he comes back and again takes to his heels when the Iqama is pronounced and after its completion he returns again till he whispers into the heart of the person [to divert his attention from his Salât (prayer)] and makes him remember things which he does not recall to his mind before the (prayer) and that causes him to forget how much he has prayed." [1:582-O.B.]

I heard Allâh's Messenger (ﷺ) saying (as regards raising the voice in pronouncing the Adhân): Whoever hears the Adhân, whether a human being, a jinn or any other creature, will be a witness for you on the Day of Resurrection." [1:583-O.B.]