Musannaf Ibn Abi Shayba 38:116

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from 'Abdullah ibn Salamah, that he heard him say: I saw 'Ammar on the day of Siffin, an elderly, dark-skinned, tall man, holding a spear in his hand, and his hand was trembling. He said: "By the One in Whose hand is my soul, if they were to strike us until they reached the palm trees of Hajar with us, I would know that we are upon the truth and that they are upon falsehood."