Musannaf Ibn Abi Shayba 28:742

Ibn ‘Uyaynah narrated to us, from ‘Amr, from Abu Fakhitah, who said: A neighbor of mine informed me, saying: I brought a captive to ‘Ali on the day of Siffin. He said: "I will not kill you by *sabran* (tying you up and killing you), for I fear Allah, the Lord of the worlds."