Musannaf Ibn Abi Shayba 18:972

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Isra’il narrated to us, from Abu Ishaq, from Sa’id ibn Wahb, who said: Some men went out on a journey, and a man accompanied them. They arrived, but he was not with them. His family accused them. Shurayh said: "Bring your witnesses that they killed your companion, otherwise they shall swear by Allah that they did not kill him." They brought them to Ali while I was with him. He separated them, and they confessed. I heard Ali saying: "I am Abu al-Hasan the Qarm (the leader/the decisive one)." He then ordered them to be executed, and they were executed.