It is narrated from Sayyiduna Anas (may Allah be pleased with him) that they mentioned fire and the bell (naqus), and they mentioned the Jews and the Christians; then 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 / 1950