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.
Reference: Sahih Muslim 378 b In-book : Book 4, Hadith 3 USC-MSA web (English) : Book 4, Hadith 737 (deprecated)
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