The Book of Prayers

Sahih Muslim

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

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...