It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that when the Muslims came to Madinah, then he narrated the hadith about the beginning of the adhan; at its end it says that the Messenger of Allah (peace and blessings be upon him) said: "O Bilal, stand and call the adhan for the prayer!"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1839