Musannaf Ibn Abi Shayba 38:93

Yahya ibn Adam narrated to us, he said: Ibn 'Uyaynah narrated to us, from 'Imran ibn Zabyan, from Hakim ibn Sa'd, who said: They leveled their spears at Siffin, and we leveled our spears. And if a person were to walk upon them, he could have done so.