Musannaf Ibn Abi Shayba 38:147

Aswad ibn Amir narrated to us, he said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Bishr ibn Shughaf, who 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 the harm they cause. As for me, I had said to them: 'Do not kill Uthman; leave him. For by Allah, if you were to leave him for eleven nights, he would surely die in his bed.' But they did not do so." "For no prophet was killed except that seventy thousand people were killed because of him, and no caliph was killed except that thirty-five thousand people were killed because of him."