Musannaf Ibn Abi Shayba 38:85

Yazid ibn Harun narrated to us, from al-Hasan ibn al-Hakam, from Ziyad ibn al-Harith, who said: I was beside Ammar ibn Yasir at Siffin, and my knee was touching his knee. A man said, "The people of al-Sham have disbelieved." Ammar said, "Do not say that. Our Prophet and their Prophet are one, and our Qibla and their Qibla are one. However, they are a people who have been seduced; they have deviated from the truth, so it is our duty to fight them until they return to it."