Musnad Ahmad 5:151

Abu Ahmad narrated to us, Sharik narrated to us, from Imran ibn Zabyan, from Abu Yahya, who said: When Ibn Muljam struck Ali with the blow, Ali said: "Do to him as the Messenger of Allah (peace and blessings of Allah be upon him) intended to do to a man who wanted to kill him." He said: "Kill him, then burn him."