Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 663
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."