Sahih Al Bukhari (476)

عن ابن عمر كان يقول {كان المسلمون حين قدموا المدينة يجتمعون فيتحينون الصلاة ليس ينادى لها فتكلموا يوما في ذلك فقال بعضهم اتخذوا ناقوسا مثل ناقوس النصارى وقال بعضهم بل بوقا مثل قرن اليهود فقال عمر أولا تبعثون رجلا ينادي بالصلاة فقال رسول الله صلى الله عليه وسلم يا بلال قم فناد بالصلاة}

Ibn `Umar (may Allah be pleased with him) narrated: When the Muslims arrived at Medina, they used to assemble for the prayer, and used to guess the time for it. During those days, the practice of Adhan for the prayers had not been introduced yet. Once they discussed this problem regarding the call for prayer. Some people suggested the use of a bell like the Christians, others proposed a trumpet like the horn used by the Jews, but ‘Umar was the first to suggest that a man should call (the people) for the prayer; so Allah’s Messenger (peace be upon him) ordered Bilal to get up and pronounce the Adhan for prayers.