Sahih Muslim Book 4, Hadith Number 737.

Chapter : Lying with one in menstruation above the waist wrapper.

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.