It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that when the people became many, they discussed by what means they could make known the time of prayer, something they would recognize. The people suggested lighting a fire or striking a bell (naqus). Then Sayyiduna Bilal (may Allah be pleased with him) was ordered "to say the adhan in pairs and the iqamah once each."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1953