Musannaf Ibn Abi Shayba 38:110

Waki’ narrated to us, from Shu’bah, from ‘Amr ibn Murrah, from ‘Abdullah ibn Salamah, who said: I saw ‘Ammar on the day of Siffin, a dark-skinned, tall old man, and his hands were trembling, and in his hand was a spear. He said: "If they were to strike us until they reached the palm branches of Hajar, I would know that we are upon the truth and that they are upon falsehood."