Musannaf Ibn Abi Shayba 1:1907

Abu al-Ahwas narrated to us, from Abu Ishaq, from Abu al-Ala, who said: Ali sent us to Siffin and appointed Qays ibn Sa’d, the servant of the Messenger of Allah (may Allah bless him and grant him peace), over us. We traveled until we reached Maskin. I saw Qays urinate, then he came to the bank of the Tigris, performed ablution, and wiped over his leather socks. I saw the marks of his fingers on his leather socks.