It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) ordered Bilal (may Allah be pleased with him) "to say the adhan in pairs and the iqamah once each."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1956