Musannaf Ibn Abi Shayba 38:124

'Umar ibn Ayyub al-Mawsili narrated to us, from Ja'far ibn Burqan, from Yazid ibn al-Asamm, who said: 'Ali was asked about those killed on the day of Siffin. He said: "Our killed and their killed are in Paradise." "And the matter will return to me and to Mu'awiyah."