Musannaf Ibn Abi Shayba 16:630

Abu Bakr narrated to us, he said: 'Abdah narrated to us, from Mis'ar, from 'Uthman ibn al-Mughirah, from 'Ali ibn Rabi'ah, who said: I saw 'Ali wearing an izar (waist-wrapper), and I saw him wearing a tubban (drawers/shorts) underneath it.