Musannaf Ibn Abi Shayba 3:378

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sakan ibn Abi Karima, from Muhammad ibn ‘Ubada, from Mahmud ibn Rabi’, from ‘Ubada ibn al-Samit, that when he would stand for prayer, he would uncover his forehead from his turban.