Hadithcore
Musnad Ahmad, 550
graded· Darussalam

It was narrated that ‘Abdullah bin Muhammad bin `Aqeel said:

`Uthman (رضي الله عنه) was killed in 35 AH and the fitnah (turmoil) lasted for five years including four months of al-Hasan`s rule.

حَدَّثَنَا زَكَرِيَّا بْنُ عَدِيٍّ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَمْرٍو، عَنْ عَبْدِ اللَّهِ بْنِ مُحَمَّدِ بْنِ عَقِيلٍ، قَالَ قُتِلَ عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ سَنَةَ خَمْسٍ وَثَلَاثِينَ فَكَانَتْ الْفِتْنَةُ خَمْسَ سِنِينَ مِنْهَا أَرْبَعَةُ أَشْهُرٍ لِلْحَسَنِ رَضِيَ اللَّهُ عَنْهُ‏.‏

References1 variant
In-Book Reference
Book 4, Hadith 142