Musannaf Ibn Abi Shayba 37:184

Muhammad ibn Bishr narrated to us, he said: Isma'il narrated to us, he said: A man who used to sell food narrated to me, he said: When Hudhayfah arrived at Jukha, he came to Abu Mas'ud to greet him. His father said: "What is the matter with this sword of yours, O Abu 'Abd Allah?" He said: "'Uthman appointed me over Jukha." He said: "O Abu 'Abd Allah, do you fear that this might be a *fitnah* (tribulation), at the time when the people drove out Sa'id ibn al-'As?" Hudhayfah said to him: "Do you not know your religion, O Abu Mas'ud?" He said: "Yes." He said: "Then the *fitnah* will not harm you as long as you know your religion. The *fitnah* is only when the truth and falsehood become confused for you, and you do not know which of them to follow; that is the *fitnah*."