Musnad Ahmad 769:48

'Affan narrated to us, Hammad ibn Salamah narrated to us, Sa'id ibn Jumhan narrated to me, he said: We were fighting the Khawarij, and among us was 'Abdullah ibn Abi Awfa. A servant of his had joined the Khawarij, and they were on that bank [of the river], and we were on this bank. So we called out to him, "Abu Fayruz! Abu Fayruz! Woe to you, this is your master, 'Abdullah ibn Abi Awfa!" He said, "He is a good man, if only he had emigrated." He [Ibn Abi Awfa] said, "What is the enemy of Allah saying?" We said, "He says: 'He is a good man, if only he had emigrated.'" He said, "Is there emigration after my emigration with the Messenger of Allah, peace and blessings of Allah be upon him?" Then he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Glad tidings to whoever kills them and whom they kill."