Musannaf Ibn Abi Shayba 37:585

Aswad ibn Amir narrated to us, he said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Bishr ibn Shughaf, he said: Abdullah ibn Salam asked me about the Khawarij. I said: "They are the people who pray the longest and fast the most, except that when they cross the bridge, they shed blood and seize wealth." He said: "Do not ask about them except for this. Indeed, I had said to them: 'Do not kill Uthman; leave him. For by Allah, if you were to leave him for eleven [days/years], he would surely die in his bed a natural death.' But they did not do so." And he said: "No prophet was ever killed but that seventy thousand people were killed because of him, and no caliph was ever killed but that thirty-five thousand people were killed because of him."