Musannaf Ibn Abi Shaybah

Hazrat Ibn Abi Mahzura narrates that Hazrat Abu Mahzura used to say the words of the adhan one by one in a low voice. Then when he reached "Ashhadu anna Muhammadan Rasulullah, Ashhadu anna Muhammadan Rasulullah," he would return to "Ashhadu an la ilaha illallah," and while saying these words twice, he would raise his voice. Then when he reached "Hayya 'ala as-salah," in the first adhan of Fajr, he would say "As-salatu khayrun minan-nawm."