Musannaf Ibn Abi Shayba 38:111

Yazid ibn Harun narrated to us, he said: Abd al-Malik ibn Qudama al-Jumahi narrated to us, he said: Amr ibn Shu’ayb narrated to me, from his father, from his grandfather, who said: When the people lowered their hands at Siffin, Amr ibn al-Aas said: The war has ignited, so I have prepared for it, with a high-withered, broad-backed [horse]. It connects the charge with a charge, and when the horses leap from the charge, it is swift. It has a large chest and a wide girth, and when it is soaked with water, it emerges. He said: And Abdullah ibn Amr said: If you had witnessed my position and my presence at Siffin on a day that would turn the forelocks gray: On the evening the people of Iraq came as if they were spring clouds raised by the side-runners. And we came to them rushing, as if our ranks were the sea, its waves surging and overlapping. Our millstone turned, and their millstone turned, throughout the day, until the shoulders turned away. Whenever I said they have turned away quickly, battalions of them appeared to us, and battalions leaned heavily [against us]. So they said to us: "We see that you should pledge allegiance to Ali." We said: "Rather, we see that we should strike [with swords]."