Ibn Umar reported:
When the Muslims came to Medina, they gathered and sought to know the time of prayer but no one summoned them. One day they discussed the matter, and some of them said: Use something like the bell of the Christians and some of them said: Use horn like that of the Jews. Umar said: Why may not a be appointed who should call (people) to prayer? The Messenger of Allah (peace be upon him) said: O Bilal, get up and summon (the people) to prayer.
(Muslim Book 4, Hadith 735)