From Sahih Bukhari (Hadith 606):
Narrated Anas bin Malik:When the number of Muslims increased they discussed the question as to how to know the time for the prayer by some familiar means. Some suggested that a fire be lit (at the time of the prayer) and others put forward the proposal to ring the bell. Bilal was ordered to pronounce the wording of Adhan twice and of the Iqama once only
Reference: Bukhari 606