Musnad Ahmad 27:1897

'Abd al-Razzaq and Ibn Bakr narrated to us, the meaning being the same, they said: Ibn Jurayj informed us, Nafi' informed me, that Ibn 'Umar used to say: When the Muslims arrived in Medina, they would gather and estimate the time for prayer, and no one would call for it. They spoke about that one day, and some of them said: "Take a bell like the bell of the Christians." Others said: "Rather, a horn like the horn of the Jews." 'Umar said: "Will you not send a man to call for the prayer?" The Messenger of Allah (ﷺ) said: "O Bilal, stand up and call for the prayer."