Musannaf Ibn Abi Shayba 38:104

Muhammad ibn al-Hasan narrated to us, he said: Hammad ibn Zayd narrated to us, from Hisham, from Muhammad ibn Sirin, he said: The number of the slain on the day of Siffin reached seventy thousand. They were not able to count them except by reeds; they placed a reed upon every person, then they counted the reeds.