Sahih Al Bukhari – Book of Virtues And Merits Of The Prophet (pbuh) And His Companions

Volumn 004, Book 056, Hadith Number 663.
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Narated By Anas RA: The people mentioned the fire and the bell (as means proposed for announcing the time of prayer) and by such a suggestion they referred to the Jews and the Christians. But Bilal RA was ordered, "Pronounce the words of the Adhan (i.e. call for the prayer) twice and the Iqama once only."