The Fitnah that Muhammad ibn Sireen (may Allah have mercy upon him) mentioned in the following narration refers to the fitnah of the murder of Uthman (may Allah be pleased with them) and the revolt of the Khawaarij against Ali and Uthman (may Allah be pleased with them):

لَمْ يَكُونُوا يَسْأَلُونَ عَنِ الإِسْنَادِ، فَلَمَّا وَقَعَتِ الْفِتْنَةُ قَالُوا سَمُّوا لَنَا رِجَالَكُمْ فَيُنْظَرُ إِلَى أَهْلِ السُّنَّةِ فَيُؤْخَذُ حَدِيثُهُمْ وَيُنْظَرُ إِلَى أَهْلِ الْبِدَعِ فَلاَ يُؤْخَذُ حَدِيثُهُمْ

They would not ask about the chains of narration, and when the Fitnah occurred, they said: ‘Name for us your men’. So Ahl us-Sunnah would be regarded, and their Ḥadīth were then taken, and Ahl ul-Bi’dah would be regarded, and their Ḥadīth were not taken

[Muqaddimah Saheeh Muslim no. 27]

al-Qurtubi said in al-Mufhim (1/122-123): “His statement: So when the fitnah occurred, they said: Name us your men”. This fitnah is, and Allah knows best, the fitnah of the murder of Uthman (may Allah be pleased with him) and the fitnah of the revolt of the Khawaarij against Ali and Mu’awiyyah (may Allah be pleased with them). For verily they [the Khawaarij] declared them disbelievers in order to make permissible their blood and wealth…he meant by that, and Allah knows best, that those who killed Uthman and the Khawaarij are clearly evil doers. Their reports [i.e. of the Khawaarij and the murders of Uthman] were mixed with the reports of other than them. So it was obligatory to investigate their reports and reject them and investigate the reports of others and accept them. Then the ruling applied to other than them from the people of innovation”

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan