(2) باب الأَمْرِ بِشَفْعِ الأَذَانِ وَإِيتَارِ الإِقَامَةِ
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. (ID)
Reference (ID): Sahih Muslim 378 b (ID)
In-book reference (ID): Book 4, Hadith 3 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 737 (ID)