From Sahih Bukhari (Hadith 3457):
Narrated 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
Reference: Bukhari 3457