Musnad Ahmad 775:20

Bahz and ‘Affan narrated to us—the meaning is the same—they said: Hammad, meaning Ibn Salamah, narrated to us. ‘Affan said in his narration: Sa‘id ibn Jumhan narrated to us. And Bahz said in his narration: Sa‘id ibn Jumhan narrated to me, he said: We were with ‘Abdullah ibn Abi Awfa fighting the Khawarij, and a servant of Ibn Abi Awfa had joined the Khawarij. We called out to him: "O Fayruz, this is Ibn Abi Awfa." He said: "An excellent man, if only he had emigrated." He [Ibn Abi Awfa] said: "What is the enemy of Allah saying?" He said: "He says: 'An excellent man, if only he had emigrated.'" He said: "Emigration after my emigration with the Messenger of Allah, peace and blessings of Allah be upon him?" He repeated it three times. "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Glad tidings to whoever kills them, and then they kill him.'" ‘Affan said in his narration: "And they kill him," three times.