Musnad Ahmad 4:14

Zakariya ibn 'Adi narrated to us, from 'Ubayd Allah ibn 'Amr, from 'Abd Allah ibn Muhammad ibn 'Aqil, who said: 'Uthman was killed in the year thirty-five, and the strife (fitnah) lasted for five years, four months of which were under al-Hasan, may Allah be pleased with him.