Musannaf Ibn Abi Shayba 38:140

Yazid ibn Harun narrated to us, he said: Sulayman ibn al-Mughirah informed us, from Humayd ibn Hilal, who said: A man from Abd al-Qays narrated to me, saying: I was with the Khawarij, and I saw something from them that I disliked, so I separated from them on the condition that I would not speak against them. While I was with a group of them, they saw a man who had come out, as if he were bald. There was a river between them and him, so they crossed the river to him and said, "We have frightened you." He said, "Yes." They said, "Who are you?" He said, "I am Abdullah ibn Khabbab ibn al-Aratt." They said, "Do you have a hadith that you can narrate to us from your father, from the Messenger of Allah (peace be upon him)?" He said, "I heard him saying that he heard the Prophet (peace be upon him) say: 'There will be a coming fitna (strife) in which the one sitting is better than the one standing, and the one standing in it is better than the one walking. So if you encounter them, if you are able to be the Abdullah who is killed, then do not be the Abdullah who is the killer.'" He said: So they brought him near the river and struck his neck. I saw his blood flowing on the water as if it were a strap of water, mixing with the water until it disappeared from sight. Then they called for a slave-girl of his who was pregnant, and they ripped open what was in her womb.