Musnad Ahmad 288:8

Abdullah ibn Bakr al-Sahmi narrated to us, he said: Hatim ibn Abi Saghirah narrated to us, from al-Nu’man ibn Salim, that ‘Amr ibn Aws informed him that his father, Aws, informed him, he said: We were sitting with the Messenger of Allah, peace and blessings of Allah be upon him, in the Suffah, and he was preaching to us and reminding us, when a man came to him and whispered to him. He said: "Go and kill him." He said: When the man turned away, the Messenger of Allah, peace and blessings of Allah be upon him, called him and said: "Does he testify that there is no god but Allah?" The man said: "Yes, O Messenger of Allah." He said: "Go and let him go, for I have only been commanded to fight the people until they testify that there is no god but Allah. If they do that, their blood and their wealth become sacred to me, except by its right."