Abdullah bin Muhammad bin Aqil said: "The fitnah lasted five years, of which four months were for Sayyiduna Hasan bin Ali (may Allah be pleased with him) (his caliphate), and the Muslim Ummah united upon Sayyiduna Mu'awiyah (may Allah be pleased with him) in the year 40 AH."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2544