Musannaf Ibn Abi Shayba 3:1303

Hafs narrated to us, from al-A'mash and al-Hasan ibn 'Ubayd Allah, from Jami' ibn Shaddad, from al-Aswad ibn Hilal, who said: Abdullah said: "Whoever lets his ears hear [what he is reciting] and does not lower his voice [is not considered to have recited]."