Musannaf Ibn Abi Shayba 2:36

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Yunus ibn Abi Ishaq, from Muharib ibn Dithar, from al-Aswad ibn Yazid, from Buraydah, from Abu Mahdhura, who said: The end of the Adhan was: "Allah is the Greatest, Allah is the Greatest, there is no god but Allah."