It is narrated from Sayyiduna Muawiyah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever dies without having pledged allegiance to an Imam, dies the death of ignorance."
Hadith Referenceمسند احمد / مسند الشاميين / 16876
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن