Virtues and Merits of the Prophet (pbuh) and his Companions

Bukhari :: Book 4 :: Volume 56 :: Hadith 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."