Virtues and Merits of the Prophet (pbuh) and his Companions
Bukhari :: Book 4 :: Volume 56 :: Hadith 663
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 was ordered, "Pronounce the words of the Adhan (i.e. call for the prayer) twice and the Iqama once only."
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