Musannaf Ibn Abi Shayba 34:187

Al-Fadl ibn Dukayn narrated to us, from Abu Isra'il, from al-Hakam, from Abu Sulayman al-Juhani—meaning Zayd ibn Wahb—from Hudhayfah. He mentioned the killing of Uthman and said: "Verily, it is the first of the tribulations."