It is narrated from Sayyiduna Anas (may Allah be pleased with him) that in the time of the Prophet (peace and blessings be upon him), when the time of prayer came, a man would run along the road announcing: "The prayer, the prayer!" This became difficult for the people, so they said: "O Messenger of Allah, what if we made a bell?" He (peace and blessings be upon him) said: "That is for the Christians." They said: "Shall we make a horn?" He (peace and blessings be upon him) said: "That is for the Jews." The narrator says: Then he (peace and blessings be upon him) ordered Sayyiduna Bilal (may Allah be pleased with him) to say the phrases of the adhan in pairs and the iqamah once.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1833