Sayyiduna Aws (may Allah be pleased with him) said: We were sitting with the Messenger of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was narrating stories to us and admonishing us, when a man came and whispered something to him. The Messenger of Allah (peace and blessings be upon him) said: "Go and kill him." When the man turned away, he (peace and blessings be upon him) called him back and said: "Does he bear witness that there is no deity except Allah (la ilaha illallah)?" He said: Yes. He (peace and blessings be upon him) said: "Take him and let him go his way, for I have only been commanded to fight the people until they say: la ilaha illallah. When they do that, their blood and their wealth are inviolable for me (and may be taken only) by right, and their reckoning is with Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 591