Musannaf Ibn Abi Shayba 2:69

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, from Muhammad, who said: The Adhan was that he would say "Allahu Akbar, Allahu Akbar," then he would place his two fingers in his ears. The first person to leave one of his fingers out of his ears was Ibn al-Asamm.