Sahih Bukhari Volume 4, Book 56, Hadith Number 663.

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.”