It is narrated from Sayyiduna Sa'd (may Allah be pleased with him) that the adhan first began in this way: a man from the Ansar was shown the adhan in a dream, and he told the Prophet (peace and blessings be upon him) about it, so the Prophet (peace and blessings be upon him) ordered Sayyiduna Bilal (may Allah be pleased with him) to call the adhan. That Ansari taught it to him like this: Allah is the Greatest, Allah is the Greatest; I bear witness that there is no god but Allah, I bear witness that there is no god but Allah; I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah, I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah. Then he repeated: I bear witness that there is no god but Allah, I bear witness that there is no god but Allah; I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah, I bear witness that Muhammad (peace and blessings be upon him) is the Messenger of Allah; come to prayer, come to prayer; come to success, come to success; Allah is the Greatest, Allah is the Greatest; there is no god but Allah.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5431