Musannaf Ibn Abi Shayba 2:70

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Hisham, from Ibn Sirin, that when he would give the Adhan, he would face the Qibla and let his hands hang down. Then, when he reached "Hayya 'ala al-Salah, Hayya 'ala al-Falah," he would insert his fingers into his ears.