‏الصلاة : Hadeeth No: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....

‏البر والصلة والآداب‏ : Hadeeth No:6262 A hadith like this has been narrated on the authority of Nu'man b. Bashir through another chain of transmitters....