Sayyiduna Aws bin Shurahbil (may Allah be pleased with him), who was from Banu al-Mujammi', relates that he heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever walks with an oppressor to help him, knowing that he is an oppressor, has left Islam."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 615