Musannaf Ibn Abi Shayba 38:105

Muhammad ibn Abd Allah al-Asadi narrated to us, he said: Kaysan narrated to us, he said: My master Yazid ibn Bilal narrated to me, he said: I witnessed [the battle of] Siffin with Ali. When a captive was brought to him, he would say: "I will not kill you by *sabran* (binding and killing), for I fear Allah, the Lord of the worlds." He would take his weapon, make him swear an oath that he would not fight him, and give him four dirhams.