Sufyan bin 'Uyaynah سفيان بن عيينة · The Book of Fighting [The Prohibition of Bloodshed]

"Killing a believer is more grievous before Allah than the extinction of the whole world."

The Messenger of Allah said: "Whoever unsheathes his sword and starts to strike the people with it, it is permissible to shed his blood."

"When 'Ali was in Yemen, he sent some gold that was still enclosed in rock to the Prophet ) said: 'Among the offspring of this man there will be people who will recite the Qur'an but it will not go beyond their throats, and they will go out of Islam as an arrow goes through the target. They will kill the Muslims and leave the idol-worshippers alone. If I live to see them, I will kill them as the killing…

The Prophet said: "Do not revert to misguidance after I am gone, striking the necks of one another (killing one another)."