We were fighting the Khawarij, and Abdullah bin Abu Awfa was among us. A young servant of his had joined the Khawarij. They were on one side of the river and we were on the other. We called out to him, "Abu Fayruz! Abu Fayruz! Woe to you! This is your master, Abdullah bin Abu Awfa."He replied, "He is an excellent man. If only he would emigrate."Abdullah asked, "What is the enemy of Allah saying?"We said, "He says you are an excellent man, if only you would emigrate."Abdullah said, "Another emigration after my emigration with the Messenger of Allah ﷺ?" Then he said, "I heard the Messenger of Allah ﷺ say, 'Blessed is the one who kills them and is killed by them.'"
حدثنا عفان حدثنا حماد بن سلمة حدثني
كنا نقاتل الخوارج وفينا عبد الله بن أبي أوفى وقد لحق له غلام بالخوارج وهم من ذلك الشط ونحن من ذا الشط فناديناه أبا فيروز أبا فيروز ويحك هذا مولاك عبد الله بن أبي أوفى قال نعم الرجل هو لو هاجر قال ما يقول عدو الله قال قلنا يقول نعم الرجل لو هاجر قال فقال أهجرة بعد هجرتي مع رسول الله ﷺ ثم قال سمعت رسول الله ﷺ يقول طوبى لمن قتلهم وقتلوه
Its chain of transmission is sahih, and Saeed bin Jahman is trustworthy, and his hadith appears in the Sunan collections.