Sahih Bukhari Volume 004, Book 056, Hadith Number 663.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 56. Virtues and Merits of the Prophets (PBUH) and his Companions
Narated By Anas : 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."