Musannaf Ibn Abi Shayba 1:1174

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-Hasan bin Ubayd Allah, who said: I passed by the bathhouse and Abu Sadiq saw me, so he said: "Do you have a waist-wrapper (izar) with you? For Ali used to say: 'Whoever uncovers his private parts, the angel turns away from him.'"