Musnad Ahmad Hadith 19149

Saeed bin Jumhan (may Allah have mercy on him) says that we were fighting against the Khawarij when a slave of Abdullah bin Abi Awfa (may Allah be pleased with him), who was with us, joined the Khawarij. They were on that side and we were on this side. We called out to him, saying, "O Firoz! O Firoz!" and said, "O wretch! Your master, Ibn Abi Awfa (may Allah be pleased with him), is here." He replied, "They would have been good people if they had migrated away from among you." They (the companions) asked, "What is this enemy of Allah saying?" We conveyed his statement to them, so he (Ibn Abi Awfa) said, "Will I migrate again after the migration I made with the Prophet (ﷺ)?" Then he said, "I heard the Prophet (ﷺ) say: Glad tidings for the one who kills them or whom they kill."