Musnad Ahmad 944:24

Yunus and Khalaf ibn al-Walid narrated to us, they said: Abu Ma'shar narrated to us, from Muhammad ibn 'Umarah ibn Khuzaymah ibn Thabit, who said: My grandfather continued to withhold his weapon on the Day of the Camel until 'Ammar was killed at Siffin. Then he drew his sword and fought until he was killed. He said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "The rebellious faction will kill 'Ammar."