Musannaf Ibn Abi Shayba 3:725

Muhammad ibn Fudayl narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu al-Bakhtari, who said: I passed by with ‘Ubaydah in the mosque, and Mus’ab was leading the people in prayer. When he finished, he said: "There is no god but Allah, and Allah is the Greatest," raising his voice with it. ‘Ubaydah said: "May Allah fight him; he is a loud-voiced innovator."