Sayyiduna Anas (may Allah be pleased with him) narrates that when the (Muslim) people increased in number, they discussed that there should be some well-known thing by which they could know the time of prayer (and gather for prayer). So they discussed that they could light a fire, or ring a bell. Then Sayyiduna Bilal (may Allah be pleased with him) was ordered to say the words of the adhan twice each, and the words of the iqamah once each.