Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 737
Anas b. Malik reported: They (the Companions) discussed that they should know the timings of prayer by means of something recognized by all. Some of them said that fire should be lighted or a bell should be rung. But Bilal was ordered to repeat the phrases twice in Adhan, and once in Iqama.