Sayyiduna Aws (may Allah be pleased with him) said: A man came to the Prophet (peace and blessings be upon him) and whispered something to him while I was listening. The Messenger of Allah (peace and blessings be upon him) said to him: "Go to them and tell them to kill him." Then he called him back, and he returned after having gone. He (peace and blessings be upon him) said: "Perhaps he says: la ilaha illallah (there is no deity except Allah)?" He said: Yes. He (peace and blessings be upon him) said: "Then tell them to release him, for it has been revealed to me that I should fight the people until they say: la ilaha illallah. Then when they say it, their wealth and their blood become inviolable for me, except by its right, and their reckoning is with Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 590