Jami` at-Tirmidhi 190

Ibn Umar narrated:"When the Muslims arrived in AI-Madinah, they used to assemble for the Salat, and guess the time for it there was no one who called for it (the prayer). One day they discussed that and some of them said that they should use a bell like the bell the Christians use. Others said they should use a trumpet like the horn the Jews use. But Umar [bin Al-Khattab] said: 'Wouldn't it better if we had a man call for the prayer?'" He said: "So Allah's Messenger said: 'O Bilal! Stand up and call for the Salat