Musannaf Ibn Abi Shayba 28:750

Narrated to us Kathir ibn Hisham, from Ja'far ibn Burqan, who said: Narrated to us Maymun, from Abu Umamah, who said: I witnessed Siffin, and they would not finish off a wounded person, nor would they pursue one who was fleeing, nor would they strip a slain person.