Musannaf Ibn Abi Shayba 2:29

Abu Bakr narrated to us, he said: Abu Khalid narrated to us, from Hajjaj, from Ata, from Abu Mahdhura: That he performed the Adhan for the Messenger of Allah (may Allah bless him and grant him peace), and for Abu Bakr, and for Umar. And the end of his Adhan was: "Allah is the Greatest, Allah is the Greatest, there is no god but Allah."