Sunan Abi Dawud Book 2, Hadith 110
I testify that there is no god but Allah, I testify that there is no god but Allah; I testify that Muhammad is the Messenger of Allah, I testify that Muhammad is the Messenger of Allah. Lowering your voice while saying them (these words). Then you must raise your voice in making the testimony: I testify that there is no god but Allah, I testify there is no god but Allah; I testify Muhammad is the Messenger of Allah, I testify Muhammad is the Messenger of Allah. Come to prayer, come to prayer; come to salvation, come to salvation. If it is the morning prayer, you must pronounce; prayer is better than sleep, prayer is better than sleep, Allah is most great; there is no god but Allah.
when the Messenger of Allah (ﷺ) ordered a bell to be made so that it might be struck to gather the people for prayer, a man carrying a bell in his hand appeared to me while I was asleep, and I said; servant of ‘abd Allah, will you sell the bell? He asked; what will you do with it? I replied; we shall use it to call the people to prayer. He said; should I not suggest you something better than that. I replied: certainly. Then he told me to say: Allah is most great, Allah is most great, Allah is most great, Allah is most great. I testify that there is no god but Allah, I testify that Muhammad is the Messenger of Allah. Come to pray, come to pray; come to salvation; come to salvation. Allah is most great, Allah is most great. I testify that there is no god but Allah. He then moved backward a few steps and said: when you utter the IQAMAH, you should say: Allah is most great, Allah is most great. I testify that there is no god but Allah, I testify that Muhammad is the Messenger of Allah. Come to prayer, come to salvation. The time for prayer has come, the time for prayer has come: Allah is most great, Allah is most great. There is no god but Allah. When the morning came, I came to the Messenger of Allah (May peace be upon him) and informed him of what I had seen in the dream. He said: it is a genuine vision, and he then should use it to call people to prayer, for he has a louder voice than you have. So I got up along with Bilal and began to teach it to him and he used it in making the call to prayer. ‘Umar b. al-khattab (Allah be pleased with him) heard it while he was in his house and came out trailing his cloak and said: Messenger of Allah. By him who has sent you with the truth, I have also seen the kind of thing as has been shown to him. The Messenger of Allah (May peace be upon him) said: To Allah be the praise. Abu Dawud said; Al-Zuhri narrated this tradition in a similar way from Sa’id b. al-Musayyib on the authority of ‘Abd Allah b. Zaid. In this version Ibn Ishaq narrated from al-Zuhri: Allah is most great. Allah is most great, Allah is most great, Allah is most great. Ma;mar and yunus narrated from al-Zuhri; Allah is most great, Allah is most great. They did not report it twice again.
He (the prophet) taught me IQAMAH (to pronounce each phrase) twice: Allah is most great. Allah is most great. I testify that there is no god but Allah. I testify that there is no god but Allah; I testify that Muhammad is the Messenger of Allah, I testify that Muhammad is the Messenger of Allah: come to prayer, come to prayer: come to salvation, come to salvation: Allah is most great. Allah is great; there is no god but Allah.
Abu Dawud said: The narrator ‘Abd al-Razzaq said; You pronounce IQAMAH for announcing the prayer; you must say twice: the time for prayer has come, the time for prayer has come. (The Prophet said to Abu Mahdhurah): did you listen (to me)? Abu Mahdhurah would not have the hair of his forehead cut, nor would he separate them (from him) because the Prophet (ﷺ) wiped over them.