Musannaf Ibn Abi Shayba 2:66

Abu Bakr narrated to us, he said: 'Abbad bin al-'Awwam narrated to us, from Hajjaj, from 'Awn bin Abi Juhayfah, from his father, that Bilal planted the spear (al-'anzah), then he gave the adhan, and he placed his two fingers in his ears.