Prophet's companion Umar proposed it
Sahih Bukhari, Chapter: Call to Prayers (Adhaan)
From: Sahih Bukhari
Chapter No. 11, Call to Prayers (Adhaan)
Hadith No: 578
Narrated/Authority of Ibn Umar
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 Apostle ordered Bilal to get up and pronounce the Adhan for prayers.