It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that when the Muslims came to Madinah, they would gather and wait for the prayers, and there was no one to announce them. One day they spoke about this. Some said: Let us make a bell like the bell of the Christians. Others said: Rather, a horn like that of the Jews. Sayyiduna Umar (may Allah be pleased with him) said: Why do you not send a man to announce the prayer? Then the Messenger of Allah (peace and blessings be upon him) said: "O Bilal, stand up and announce the prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1831