Musannaf Ibn Abi Shayba 35:310

Abu Bakr ibn Ayyash narrated to us, from Abdullah ibn Sa’id, from Khallas, who said: I saw Ali urinate in the Rahbah, then he wiped over his socks and his sandals. And it was mentioned that Abu Hanifah used to dislike wiping over the socks and the sandals unless their undersides were made of leather.