Musannaf Ibn Abi Shayba 38:109

Abu Usama narrated to us, from Hisham ibn Urwah, who said: Abdullah ibn Urwah informed me, saying: A man who witnessed Siffin informed me, saying: I saw Ali go out during one of those nights, and he looked at the people of Ash-Sham and said, "O Allah, forgive me and them." So Ammar came, and he mentioned that to him. He (Ammar) said, "They have drawn for him the *hatir* (the enclosure/boundary) just as he drew it for you," meaning Sa'd, may Allah have mercy on him.